Try again, as one of your colleagues suggested, take out one RAM chip from two and work. Run memory-hungry programs. It sometimes happens that MemTest does not catch an error or it catches it after a long-hour test.
I will not forget a situation where, like you, the system was "almost" stable, and starting eg Adobe Photoshop demo resulted in bluescreen. Plus, the computer had been working fine all day - I mean surfing the web, writing documents, and so on. Eliminating that memory chip solved the problem.