which memory test did you use?

there are only two memory testers I trust, since they never have been wrong:

http://freshmeat.net/projects/memmxtest/?topic_id=146
http://freshmeat.net/projects/memtes...c_id=861%2C146

The first one, memmxtest, is a bit tricky and has a very complex CLI, but reading the README should answer most questions.

The second one, memtest86, turns off caching in some versions so you'd better turn it on again before you test, because sometimes caching plays tricks on your results.

If both memory testers run perfectly, that doesn't necessarly mean that your memory is 100%ly error-free, but at least it shows that your memory is 99.99995% error free - which probably won't show you one blue screen at all (remember: most "natural" memory errors occur in dependencies - simple example: if memory cell 1 is on, memory cell 2 contains a false information.). If you have ECC chips, you even have less errors, but usually there are only 2 states of memory chips: the ones which work and the others which don't. sometimes memory chips also are incompatible with each other, they might work on their own, but won't work together. in other cases, the clocking makes some memory chips unuseable.

huh, more text than I indented to wrote, but maybe it's a good candidate to refer to if somebody has a similar problem :)