This happens every single time. It gets close to the end of the loop and drops out. My UAD is set to 2048, I've set the IO buffers to 512 - nothing fixes it. I have an absolute monster of a machine and this should not happen.
 
I searched here and on the web and see similar issues dating back to 2006! I've kicked this to Cakewalk support but I'm hoping someone here can help. I am just about to give up and go back to Ableton... :/

I had dropouts with a MOTU 828x with Sonar Platinum no matter what I changed(even though the MOTU worked fine with other software)... I switched to a RME Babyface Pro and those problems disappeared. I loop sections and use groove clip looping all the time... I can run at a 64 buffer size and still work without glitches, although I mostly use a 256 buffer size in the RME. You may want to run LatencyMon to check if there are any drivers or hardware causing unwanted latency. My machine is really powerful as well, but I had to disable my Ethernet card to get everything to run smoothly and show good results on LatencyMon. With an Edirol UA-25EX interface, I had to actually lower the buffer size to eliminate dropouts. Hope this helps... I've never tried UAD on a PC before.

Thanks Mark - I'll check out LatencyMon!

Many hours of trouble-shooting and I found the cause: Output Movement.
 
I've submitted a trouble ticket to Output and I'll post what I find. I'm going to let Cakewalk support know as well...