I've got right now two mobos to play around with my Celeron 566: Abit BH6 and Aopen Ax63Pro.
With a QDI (I think) slotket, neither of both will run 850 (100fsb) stable with less than 1,75 volts.
With a standard heat sink and a bigger fan it seems like it does't run too hot, since it's stable (I can't measure temperature).
You didn't tell what Vcore did you try, but that´s the one that worked for me.
Since bus you fsb is set to 90 it does´nt seem to be a PCI problem if your divider is set to auto (changes to 1/3 at 83, I may be wrong). Bios?
I burned another Cel566 that also needed 1,75 volts to run 850, but...wait, I did burn it at 3,5 volts when I wrong-switched the slotket at 4 a.m. in the morning!!! (watch out, terrible experience!).
Saludos a todos