WHAT IS THE BEST, MOST STABLE A/D D/A INTERFACE FOR SONAR X3? 
I'm having very little success with the Roland Quad Capture or Focusrite 8i6 interfaces.  I've tried to get the latest driver from Roland but the newest driver does not appear in the downloaded file.  Tried twice.  
Focusrite 8i6 is using the latest driver and firmware versions but Sonar recognized it for only about 3 minutes before Sonar lost all sight of it.  My computer no longer sees it either.  Working with the 8i6, I've replaced the USB and power cords. I've tried other USB ports on back and front.  Of course, I have only one interface connected, and it's driver installed, at one time.  I have 2 exterior hard drives connected directly.  
 
Is there an A/D D/A interface that works properly?

If you can't get what is considered 2 of the most stable audio interfaces to work, you won't get any interface to work. You don't happen to have asio4all installed do you? 
Sounds like other audio drivers taking over to me. 
Also read Craigs advice on Hi Def audio in your Video card. 
http://forum.cakewalk.com/FindPost/3105274
 
 
 

Either one should work fine, a little more information about the rest of the machine might help.

I agree with the statements above. Both are fine interfaces and widely used. Unless you happen to have two broken devices, searching for another interface will probably just be an exercise in further frustration.
 
My experience with Focusrite support has been absolutely superb. You might want to reach out to them for advice.
 
A number of things can interfere with USB-connected interfaces. Top of that list is the USB port and USB cable, which you've already vetted (but don't forget about the internal cable and connector!). Make sure your power-conservation settings exempt the USB ports. Powering down ports when idle is a common reason for USB-connected interfaces to suddenly stop responding. Also go into your Device Manager and disable the onboard audio interface that's built in to your computer's motherboard, as that can sometimes be a source of conflict.
 
Keep in mind that SONAR doesn't actually "see" your interface. Everything it knows about the interface it learns from Windows, and everything Windows knows it learns from the driver. Verify that Windows knows about the interface, via the Sounds control panel applet. If the interface doesn't show up there, it won't be available to SONAR. If it's not there, then the driver has likely taken itself out because it failed to initialize or talk to the interface. Re-installing the driver sometimes fixes it, but if you've got the wrong driver for your O/S that won't help. You might try using WDM/KS instead of ASIO. Vendors are sometimes better at writing one over the other.

Can you see the devices in the windows device manager or do you see an unidentified device? If you can see an unifentified device it is probably the audio card and you could install it there. If you cannot install the device this way you could remove the unidentified device and restart the computer and then try installing the normal way. I have done so.

We can assume you're not using a USB hub, correct? You may also consider removing as many other USB devices as you can (just as a test) to see if it helps.

Thank you for your help.  It turns out to be the USB cable that was bad so, it looks like I have the Quad Capture up and running except for one thing:  The sounds all sound muffled, like they're coming through low pass filter; almost no high end.  I have this problem with my Sonar sound and also with YouTube videos so I know it's some setting in the Quad Capture that needs changing.
 
Any suggestions, please?

Is there an eq thingy for the interface? It might have a wrong setting. Also if there is a reset button for the interface you could try it.

AMAZINGLY, ONCE AGAIN (!) it was a bad cable.  The problem, at least with the sound, is solved.  I'll keep you informed of my overall success.
 
Thank you all very, very much for your help!

Johnnys Tip of the day: 
Troubleshooting audio issues 
 
When ever you get a hum, weak signal or just something not right. 
If it's only one side of a stereo set up start swapping cables starting and the speakers and working backwards. When you find the point were the issue doesn't switch channels you've found your bad cable or device.
 
This works real well on a PA system. By swapping I mean using the same cables but change from left to right.
In a mono audio signal path we swap out cables and devices working backwards. But in a stereo audio path swapping channels speeds up the process. 
 
No sound from Left speaker:  
 
1. Swap speaker wires at speaker and issue swaps= Speakers OK  move to #3  
                    Note:  now our right speaker is dead
2. Swap the speaker wire at speaker & issues doesn't swap = L Speaker faulty 
 
3. Swap speaker wire at power amp and the issue swaps= Speaker wires OK 
                   Note:  now our Left speaker is dead. Move to # 5 
4. Swap speaker wire at power amp and the issue stays= Speaker wire faulty  
 
5. Swap input cable at the power amp &  issues swaps = power amp OK 
             Note: now our Right speaker is dead. Move to # 7 
6. Swap input cable at power amp and issue stays = Power amp faulty 
 
7. Swap output cable at mixer &  issues swaps = Cable OK 
            Note : we are now back at Left speaker dead..no need to move any cables. 
                Mixer is source of issue check mixer settings. 
 
8. Swap output cable at mixer & issue  stays = Cable faulty 
 
One thing comes to mind is how using Powered speakers eliminates 2 places where the system can fail, the power amp and the speaker cable are eliminated. Downside is when the combined unit fails (both speaker and power amp) you have lost one side. 
 
Above method can also be used when troubleshooting a home studio system. Always work backwards from the speakers. Difference will be at the audio interface we then have a USB cable, then possibly a USB Hub, then USB ports to contend with. But the logic is the same. Work backwards and don't rule out anything. 
 
This also works from the input side of trouble shooting too. Keep working towards the source.  
Take the above situation where no sound was coming from the left one step further and all checked out back to the mixer.  Well the example is not good because any idiot could have told you the left meter wasn't moving so all the above was pointless. But that said if the left meter was moving and everything else checked out you have a faulty output jack or Left fader. 
But anyhow- lets say all mixer settings are correct but no sound from the left. 
Your input was an MP3 player to test.  
Swap the red /white RCA inputs and the issue moves = cable is OK etc