Ok.. So I am back. I spent the weekend working on this. The problem still exists. Here is what I did (apologies for formatting coz I am using my iPad to type - wifi disabled on laptop).

1 - after last post, the drop outs still continued. I ensured all unwanted devices were disabled in "device manager" - wireless, on board sound card, etc.
2 - uninstalled Asio v2 so I had only tascam asio drivers. Problem still exists.
3 - Did some research and tried finding Rattv3 for win7. Could not find it. But found LatencyMon. Ran LatencyMon to find the possible problems. Following are the discoveries...
4 - istor.sys: Intel Rapid Storage utility was flagged. Removed it from startup and disabled associated services. Problem still exists.
5 - dxgkrnl.sys: Nvidia graphics driver was flagged. Changed options in Nvidia control panel from "quality" to "performance". Problem exists.
6 - usbport.sys: I ensured my USB driver is latest. Ensured that win7 sp1 USB fix is installed. Things wok fine until suddenly the audio stops. Usbport,sys is usually shown as the culprit. But I get i8042prt.sys also sometimes with high latency readings.

Now I am stuck and not sure how to proceed. Any tips?

*edit* - the laptop seems quite well behaved until I stop Sonar midway during playback. So I start LatencyMon. Everything is fine. Then I start sonar. Everything is fine. Then I open a project. There is a latency problem. I disconnect and connect my tascam. Things are good. No audio glitches. Then I stop somewhere midway to make some edits. Go to start of song... Latency problem again (as per LatencyMon ) and audio stops working.
 
*edit 2* - I used up all my log weekend time to figure out what may the problem. Tascam WDM drivers work. There is some occasional stutter, but the audio recovers. When is ASIO mode, once the audio drops out.. the only way is to unplug the tascam and reconnect it.

Anyone?

OK. Looks like there is no more help out there. To close this discussion, I tried another few days to figure out which of my processes was responsible for the latency to the extent that my system started getting quite unstable.
 
I have now re-formatted and reinstalled the whole of Sonar (a good 2-days worth of work). The WDM drivers for Tascam seem to work. I get fairly good latency of 17 ms. But the ASIO drivers are really really unstable. While I get latency of 5-9 ms, the audio keeps dropping out and the only way to recover is to remove and re-connect Tascam to the system.
 
My conclusion: Either Sonar does not like Tascam in ASIO mode OR Tascam just seem to have crappy drivers.

Have you configured enough audio samples in the ASIO properties?
Do you have an SSD or a hard drive? 
 
If your computer works fine the chance is the problem is with the audio interface.
 
On Windows I can only recommend 3 good habits:
- regularly delete most everything in your temp folder (Windows-R, %tmp%, Ok) just after the start
- Do all windows updates manually to be sure they won't be downloaded and installed untimely.
- And of course, if and only if you have tradition hard disks, defrag them at least once a week.
 
Ideally, you would find someone to lend you a recent audio interface. Or you could perhaps rent one in an audio shop. I can vouch for example for the Roland Quad-Capture. You would then know whether it's an audio interface problem or a computer problem.

It has been said before in other threads that past Tascam drivers are of questionable quality.  Craig assures us that they have improved for the new products since they started writing them in house.
 
If WDM works and you can live with the latency, then use WDM.  The point of ASIO is that it's low latency by bypassing most of the Windows subsystems.  But the difference in latency you're seeing between WDM and ASIO is the cause of your problem, ie: something else in the laptop isn't able to keep up....
 
Try setting the audio buffers to a high level (long latency) and then slowly reduce until you start getting dropouts again. 
Try a test project that doesn't use the disk, ie: several tracks of synth (not using samples) like Pentagon. Put lots of FX on the tracks to further load the CPU.  (Doesn't matter what it sounds like, you're just load testing)
Try a test project using large samples.  If you have Kontact use several of the synths with big sample files.  The intention here is to force lots of disk activity, which I think is going to be the problem.
 
Laptops generally have cheap, slow HDDs.

Thanks Cedric, Karyn for your responses.

I am quite a PC-maintenance freak so most points that you have mentioned Cedric is already being done. I have a scheduled defragment every Wednesday night, my temp folders get deleted every time my machine starts up, and I HATE windows Auto updates. So I only install them manually after ensuring if I really need them. I don't have an SDD. But my drive is 7200 rpm. Should be good for most cases.

Karyn - I agree with the disk activity. As part of debugging, I have only 3 tracks - one audio, one cakewalk tts and one rapture instance. My machine is not able to keep up to that too. What intrigues me is that as I mentioned before, it used to work up until platinum 1st release. Audio used to drop out. But I used to get a message and restarting the audio engine used to do the trick. So I am inclined to believe that while my laptop may have issues - that's not the whole story. After installing Dorchester release, I have stopped getting the audio dropout messages and the only way to get the audio back is to disconnect and reconnect tascam. I have verified this as part of my re-install.

So I am wondering if something changed in Dorchester (possibly and un-intended butterfly effect of some change) that may be causing this behaviour.

In any case, I'll scout to see if I can get my hands on some other audio device with asio driver so I can conclusively rule that the issue is with Tascam driver.

Will keep the forum posted 😊.

i was going to chime in about the drivers, as i used to use Tascam devices (us-122l & us-144) and their 64 bit drivers were a nightmare on my Windows 7 x64 AMD system.  these devices worked like a charm on 32 bit operating systems for me, but once i switched to 64 bit, it was a much different story.  in retrospect, i can't even begin to fathom how much of my time was wasted rebooting my computer & researching potential fixes for my problems (which seem to be much worse than the ones you are experiencing).  but seeing as this is a discontinued device, i doubt the drivers will ever be updated.  i sold my Tascams on Craigslist a couple years ago & got a Roland Quad-Capture & have been very happy with it (and spend much more of my studio time actually making music)

😊 thank you for responding WetDentist. It is consoling to know there are others out there. I will most likely start shopping around for another card. Will look into Roland Quad Capture.

FWIW, TASCAM's drivers do not play nice if ASIO4ALL or the "HD Audio" driver some graphics boards generate are installed. You probably know this but might be worth checking before giving up.
 
Before switching over to the US-4x4, I used a US-366. I don't know if the drivers are similar to the US-122 Mk II but found that it would be problematic under the conditions listed above. Once I got rid of those drivers it worked fine in a 64-bit Windows 7 system.

Hey Anderton,
 
I do have the NVIDIA and IDT High Definition Audio drivers on my system. I tested Tascam by disabling them from Device Manager. But the problem persisted. So yesterday I went out an loaned an RME UCX . I know there is no comparison between the Tascam and RME, but the UCX performed at 192Khz with 0.3ms latency and buffer size of 64 without any glitches. The laptop also did not seem strained to do its processing. I pulled out an old project with 6 VSTs and 4 audio tracks. This used to stutter with Tascam every 5 sec. But the whole song played with UCX without a glitch.
 
I am now sold on UCX and will most probably get this product (have to save up for it now ). But one thing good that has come from this exercise is that I managed to clean my laptop off all the unwanted drivers and start-up tasks .