Hey there Semper fi!
I had the same problem on a dualboot system running on a Tyan 1854S Trinity board. Would run 50% o/c'd (500E running 5x150 @750 MHz) dead stable on the Win98 side. Booting into Win2K was a nightmare until I slowed it down to near default. Running the same CPU on a CUSL2 with Mushkin RAM, no problem running Win2K at 750. Strange, not? And to make things easier on the dual or triple booting options, System Commander is a nice solution.