Reply 20 of 25, by shamino
The P2B boards were built with good caps, so it's unlikely to be those (even though they are old at this point).
Years ago when I used to get bulk lots of motherboards, I occasionally ran into boards that had this kind of behavior (working only when underclocked). Not sure why it happens.
A worrying thing is that it might continue to degrade until it's not stable at any usable speed. 🙁
I wonder if some aspect of the power circuitry somewhere is failing, like a dying MOSFET or something. The tests you've done don't show any obvious problem with voltages though.
One possibility (no real evidence, just a random guess) is that it could be a function of power draw from the Vcore circuit. If that were the case, then it's possible a CPU that draws less power would be able to work at a faster clock.
I'm not really advocating spending more money on random parts.. but do you happen to have a low end Coppermine? These boards frequently support Coppermine voltages but it depends what regulator chip you have. It's the ~20pin chip between the LPT port and the CPU.
Alternatively - does your P2-450MHz have a partially unlocked multiplier? Some Deschutes chips will let you set the multiplier lower, but not higher. Might be interesting to see if it can run at 100FSB with a lower multiplier like 3X or something. But I think most likely it's multiplier is locked.