OK, a couple of tests:

_STRIDES: solo piano runs pretty cleanly from 85 - 280 or so
M_JBSBPD: minor artifacts start at around 95 (apparently on bass track) but don't get out of hand. The piano has no problems.

So I guess I'm just overtaxing the real tracks by trying to play them out of their normal tempo range. Good to know, and it shouldn't affect performance; I just need to accommodate that in my practice regimen.

Thanks for the help, guys.