Need Help Understanding OC results for 'old' Celery not liking Win2K

I've got an old ABit ZM6 (PPGA360) MoBo and picked up an old Celeron
566 coppermine (cB0) which the board DOES support w/ last couple BIOSs
(it's a PPGA board and this is an early FCPGA chip). Previously I had
a 366(66) OC'd to 550(100) for years no prob. I dropped in the 566 and
w/ 66FSB sure enough it works just fine at 1.5V as advertised.

Now here's where I've run into some rather odd behavior given my
limited experience w/ OC'ing - I suspect it may just not be OC'able
but I'd be curious to understand it's behavior. Is there something
special about Win2K at boot time that taxes a CPU perhaps?

Basically it will post 850mhz all day long about 1.75V to 1.80V or
better. It will boot into DOS(w95) easily as well using a boot disk.

Here's the odd behavior: It is a ONE-TIME run session immediately
after a BIOS Flash for Win2K specifically.

It will never reboot again into Win2K. Sound strange? Let me explain
further. Let's say I set the voltage to say 1.85 or 1.90V default
(those are good popular voltages for an OC'd SL46T) - to do that I
have to reflash the BIOS so the Abit will give me a proper 'default'
voltage range. Immediately (and 1 time only) after a given reflash I
can then boot it ONE time and ONE TIME ONLY at 850mhz(100FSB). That's
IT! It will never reboot, restart or power-off and get back into Win2K
at that speed. Doesn't matter what I do to modify the voltage further
(I've gone as high as 2V). It will hang on booting into Win2K next go
around period.

I CAN get back into Win2K if I do a reflash of the BIOS as I would to
say modify the default voltage. I am then granted one single full boot
into Win2K. BUt I know of know other 'trick' to allow me back in a 2nd
time...

I CAN also get back into Win2K if I drop the FSB down to 66 for a
stock 566 speed (and of course old DOS at 850mhz too).


Once in Win2K it's golden at about 1.85V to 1.95V - any of those will
yield seemingly long term stability and hours and hours of Prime95 w/
temps as high as 39C (1.95V). I can play games, do nothing, whatever -
seems rock solid.

But remember I CANNOT get BACK into Win2K a SECOND time if I do a
restart or power it down.

And also remember that this machine has run for years at 100FSB w/ a
366 OC'd to 550 under the same Win2K setup. (DO I need to do something
special in Win2K when upping to a new CPU?)

That just doesn't seem consistent w/ my experience w/ OCing. Even
though I can trick it into 850mhz under Win2K thru the hassle of a
reflash each time - obviously it's not worth the hassle so I'd have to
say this one 'apparently' is not OC'able. But I'd like to better
understand it's behavior as well as whether or not there is something
about Win2K or other BIOS settings etc I should be aware of here.

thanks!

PS: Also the case reset button no longer functions w/ it set at
100FSB(850mhz) - usually I have to power cycle now too...?