Until yesterday my XP2000+ was happily running at 1667mhz but then it dropped to 1250mhz. My FSB remained set at 133mhz and even increasing it to 166 failed to convince the processor to return to its previous speed. My system uses an MSI K7N2 mainboard with bios version 3.9, 512mb of Patriot PC2700 DDR ram.

Sounds like you bios reset to default 100 fsb. And if you set it to 166 it probably reset itself again. If the post screen does not indicate 133 it is probably at 100 again. Do you have a software recognition program such as cpuid, sisoft sandra, or aida32.

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( Quote )
Possession of a powerful computer is no substitute for a working brain.
A working brain is no substitute for possession of a powerful computer.

It may just mean your CMOS battery has crapped out, try replacing it and setting 133 FSB again.

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AMD Athlon 64 3200+ Clawhammer
Gigabyte K8V800MAE K8T800 Mainboard
2x 512 MB Kingston Hyper X DDR400
PNY Verto GeForce 6800 GT 256 MB