It could also be one stick of PC100 ram that is of very poor quality so if by chance the bios is set to access the RAM at a latency of CAS2 then the other sticks may work but this one may be unstable. Should check the BIOS and make sure CAS is set to 3 and not 2 just to keep things safe.
I had that happen but I play around so it was me setting it to CAS2 to begin with.
