My first suspect would be faulty RAM, but as you've run memtest86 ... and it showed no errors, the RAM is probably ok.

Perhaps one of your HDs is on the way out?

I've found that HDs sometimes don't fail "instantly" but become gradually worse over time before letting go completely.

A common symptom is for the HD to shut down while being used. If you have the case open you can usually hear this (assuming there aren't too many fans). A power cycle usually coaxes it back into life (at least temporarily).

Quite why this would result in BSODs I don't know (usually just makes the system hang waiting for the failed HD to recover), but maybe there is some data corruption (which should theoretically never happen)

I'd take extra care of your backups if I were you - Make backups daily and test the media.