Horrible dropout problem!!

I've been using X1 Producer for years with little to no issue. All of a sudden it's become almost useless. I can't play or record more than a minute or 2 of a session without a dropout. However, I have no issues with Studio One. And -just to test it - I downloaded a trial of Reaper and tried it - no problems. It just seems to be Sonar.
 
Here's what I've tried:
Opening a session in safe mode (no plugins)
Swapped audio interfaces (from my normal Profire to a USB Line6)
Reinstalled Sonar (to same hard drive)
Reinstalled Sonar (to a different hard drive)
Changed buffer settings
Uninstalled antivirus
Closed ALL programs running in background
 
Nothing has worked. However - one thing did seem to work. I created a new session and imported about 10 audio tracks (from a previous mix). I set it to loop all night and it was still going when I woke up. I then lLOADED it with plugins and it still seemed to work ok. So I don't know what that means.
 
I have a pretty beefy computer built mostly for audio (i7-2600K, Gigabyte board, 16gigs ram, TI chipset for FW). I don't see any odd spikes in Sonar's Performance window. I'm running Win7 home 64bit.
 
I finally decided to post here in the hopes that someone has an idea. Thank you!
 

When you reinstalled Sonar, did it retest your audio card or just reload the previous settings? Try running the wave profiler 
 Go to Edit > Preferences > Audio - Driver Settings and click the Wave Profiler button to test your audio hardware. 

If you toggle off/on your effects - by hitting 'E' on your computer keyboard, does this issue then go away?  (turn effects back on after testing, by hitting 'E' again).
 
If the problem vanishes without effects being on, then one or more effects is adding a bunch of latency.
 
Also, check your ASIO Buffer Size, make sure you are indeed running in ASIO driver mode, and try a blank new project with a small ASIO Buffer Size (I use 128 for recording - though I could also use 64) and no effects plugins, and see if you can incrementally add things to the test project to get a better handle on when things start to go South.
 
Bob Bone