well, silly me. it seemed to be a latency issue, but this is weird and maybe something to take note of if you do these things.

with a bit more experimenting, it didn't seem to be any specific plugin. if i unfroze 2 or 3 of about ten running simultaneously, it would run fine. so, i'm back to cpu or ram. except that doesn't make sense because one freezes to open up cpu and ram.

i noticed that some of the frozen tracks were frozen at different latencies. the drums were frozen at 256, but i had to increase the latency in the process of finishing the thing, so i froze the bass at 736.
what i did to fix it was to go back and make sure everything was frozen at 256. then, when i played back at 736 it was smooth.

i'm not sure how that makes sense. maybe what was happening was that i was getting some kind of wave interference on the playback, due to the sample rate.

whatever it was, it's working now and i hope to get some work done today.

what i mean is that the files may have been out of phase, and what seemed like a stutter was actually a cancellation.

just a guess...