i've had similar problems in the past. put two sticks of ram in a particular machine and whoops. take out a stick, it works. swap the sticks around, it works. swap the slots over, it works. put both sticks in another machine, it works. put back the two sticks in the awkward machine and whooops, blue screen when it feels like it.
there was a slight difference to your situation and that is neither stick displayed any errors when running ram tests using dos based scanning software. i just accepted it as just another of those weird pc problems that don't make any sense.