Have you tried using ASIO as a driver mode?  
 
Also, I used to use AVG, but I found that the version I was using interfered with audio processing, causing a bunch of latency issues, so I switched to Avast and have been happy ever since.
 
But, I would suggest you try specifying ASIO for your driver mode first and see if that helps clean up the sound.  You could additionally temporarily disable AVG during audio processing sessions with Sonar to see if that helps, as well.
 
Bob Bone
 

If you do change to ASIO - try a sample rate of 44.1, a bit-depth of 24, and an ASIO Buffer Size of 128, and see how that works.  
 
It is also recommended that when you change your audio interface settings, that you also power down the audio interface and back on, to load it with the new settings.  Also, please note that your Sample Rate in Sonar should exactly match the sample rate you have set for your audio interface.
 
Bob Bone
 

Checked all my settings, sampling rates match in Sonar and at the hardware level, and shut off AVG.  Still have latency issues but are slightly better.
 
I did a low-tech fix to the drums on two projects I was having latency issues with....  I just deleted a few milliseconds off each one, and moved it over to line up with the time line and it worked fine.  :)

I did also notice something strange.  I get more latency problems in the projects I imported from X1, than the projects I newly create in X2.   I don't know if that's supposed to make any sense, but it's something I noticed.  wierd

There are two sources of latency, your audio interface settings and certain plug-ins. Check your existing projects for latency inducing plug-ins and bypass them. Look for plug-ins like LP-64 EQ, LP-64 Multiband, PC Concrete Limiter, R-Mix, any other mixing/mastering plug-ins with look-ahead buffers or convolution reverbs like such as Perfect Space.

So - let me try again - please post back with which driver mode you are using.  (ASIO?  WDM?)
 
Also, have you tried running DPC Latency checker - without Sonar running?  This would give you an idea of whether or not there are system-induced latency issues.  If present, then you may need to go through a careful examination of both hardware and software that is running, to see what is causing the issues.  Latency Mon is also a good tool for this.
 
AND, do you have these latency problems when you bypass all effects?  (hitting 'E' on your computer keyboard toggles all effects off/on).  If your latency goes away when the effects are bypassed, then you have one or more plugins causing your latency issues.  If so, then you need to figure out which plugins are causing this to happen.
 
Please do post back on your driver mode setting, and also review and respond on the above.
 
Bob Bone
 

I am using WDM drivers - to my knowledge Tascam doesn't make ASIO drivers for the 600.
 
Forgive my ignorance, but I've never of DPC Latency checker or Latency Mon.  Where do I find those apps?

DPC checker
Latency Mon
Both first hits in Google search

Sure - these are freeware tools that help determine the ability of your machine to do things like audio processing with Sonar.
 
Here is the link to DPC Checker:
 
http://www.thesycon.de/deu/latency_check.shtml
 
And to Latency Monitor (Latency Mon):
 
http://www.resplendence.com/latencymon
 
Give the above a shot at running for several minutes (use 1 at a time), following their doc on usage.
 
They will let you know if your system is causing the latency, either through drivers or applications running in the background, etc.
 
By the way - your specifications for the interface DO say that the interface supports ASIO.  Here is the link to the Tascam product page for it:
 
http://www.tascam.eu/en/us-600.html
 
If you scroll down to the specifications, you will see it supports ASIO as a protocol.
 
I would think that you may well have the ability to specify a driver mode of ASIO for your Tascam audio interface, which will LIKELY (not guaranteed, but quite likely) give you better audio performance.
 
If you ARE able to specify ASIO as the driver mode, choose it, then:
 

Click ApplyOK your way back out of PreferencesExit SonarPower off/on the audio interface (may just have to disconnect USB if no power switch available)Restart Sonar and test it out

 
Bob Bone
 
 
 
 
 

Did the latency and buffer settings in Sonar revert back to factory, assuming you may have tweaked them before the re-install? Silly question, maybe.

Bob, thanks for that heads-up.  I downloaded DPC Latency checker and I'll run that.   I'll also try running Sonar with the ASIO drivers -- I assumed that they already got installed because Tascam only lists one set of Win7 drivers I think?
 
Also....I've noticed that I have much more latency with X1 projects that I've moved into X2 than with projects that I've newly created in X2 -- even after I have A) disabled antivirus and startups and B) after I've ran the Plugin scanner and Wave profiler.
 
Why is that?   Any tips for that?