PC400? What type of RAM are we talking here? If this is RDRAM, then you need to make sure both sticks are functioning, and that one isn't dead, as that would cause a massive error. (although, RDRAM starts at PC600 I thought....) If this is SDRAM, SD is notorious for heat damage, so be wary of that... If it is DDR, and you were refering to PC3200 which runs at 400MHz, then again check RAM for capacitor errors, since that could cause a bigtime problem.