Sounds like your having problems with the power supply and or motherboard not able to deliver enough amperage to the CPU or even possibly memory timings set incorrectly within the motherboard BIOS. Have you tried declocking the processor and the front side bus i.e. going from 266MHz to 166Mhz or from 166MHz to 100MHz etc by going into the BIOS and slowing doing the front side bus speed. The processor multiplier should be kept the same. This may make your system more stable and reduce the power that your PSU can't deliver. Also make sure your BIOS is setup for Plug and Play OS if running Windows XP. Normally you can get into the BIOS by pressing F1 F10 or ESC etc (depends on the motherboard make) just after booting the machine.

Making sure your motherboard drivers are up to date for the motherboard chipset can also help especially if they are defaulting to the generic Microsoft drivers.

There was probably nothing wrong with the memory, more likely the BIOS timings were set to aggressively.