Personal experience: DDR is VERY fussy!
I bought a stick of Micron 256mb PC 2100, Then bought another stick 2 weeks later. Both were identical, (except the lot #s), and from the same store.
But I had nothing but weird anomolies, lockups, and BSOD's untill I traded them for a pair from the same mfgr run.
Allway update yer DDR in SETS. Don't add to it. The closer each stick is to being identical to the rest, the better!
S!