Stuttering soft synths in loop?

Win 7, I7, 16 gigs of ram, sonar X1:
I loop a bunch of soft synths for a couple of measures.  As it loops, it stutters more and more with each loop.  Stop and restart, problem goes away, but starts stuttering again with each iteration. Doesn't happen when I play a long section without loops, but with the same soft synths.   CPU and ram meters are barely budging. 
Any ideas?