have LimpidClock. It runs at pri -20. Increasing (not decreasing) LimpidClock's priority to *above* the CPU heavy task almost removes all stuttering!!!
I have a theory:
LimpidClock has a 1 second timer. It wants to run once a sec. Once in a while (when the CPU heavy waits for disk I/O perhaps) it gets to run for a brief moment, long enough to start a graphics operation and lock the system draw semaphore. Then it doesn't get to run again until the next second. During this second the CPU heavy task runs and the system, including mouse and keyboard are frozen.
I don't know what may have chnaged, but if that is remotely the case the Limpid design clock is very broken, you should not hold such drawing locks for an extended period. I'd guess it would be a calle to p96LockBitmap() or somesuch. I know from experimentation with SRec src that hold that lock even for very short periods can cause the mouse to appear to jitter slightly.
Solution is simple remove broken sftware like limpidlcok from your system.