Recently I replaced the motherboard and processor in an old XPS333 I had with a 800MHz processor and motherboard from an XPS800r. Other specs are Matrox Millenium II graphics, 80GB HDD, CD-ROM, & 128MB RIMM.

 

Initially the machine ran well, however yesterday it crashed and wouldn't sucessfully restart. I tried swapping the processor with a 600Mhz slot 1 133MHz FSB processor which didn't help, however an older 400Mhz P2 100MHz FSB works perfectly. I flashed the BIOS to A08 and removed and replaced all connections but the situation remains. With either of the 133Mhz FSB processors the machine will usually boot (and the BIOS recognises them correctly) and Windows XP will usually load but crash as soon as it's asked to do anything "strenous" (like run PCMark04 or rip a CD using iTunes). With the 400Mhz 100MHz FSB processor in place the machine is stable and rips CD's and runs benchmarks with no problems.

 

Any suggestions for what the problem could be?