It's possible, but I think it should work.. At that speed, your PCI (and drives and such) should be runing at only 3mhz above spec. I've got a 2x toshiba drive that works great with as much as 9mhz+ pci speed.
It could possibly be unstability of the processor.. When my p3-650 was oc'd to 866, I had some problems running certain video files (mpeg4, divx), but worked fine when lowered a bit (800mhz). It passed all other overnight stability tests at 866..
This is only one possibility.. I'm sure there's other things it could be
