I'm trying to get my desktop machine stable. It blue screens twice a day currently

Intel Pentium m760 (133MHz FSB)
DFI 855GME-MGF (CPU-Z reports revision A2)
2x Kingston KVR266X72C25/512
I had this system running perfectly stable for 5 months until I added an extra 512MB RAM. An now it crashes twice a day. I bought a replacement memory stick for the second slot but still have the problem.
I found an option in the bios that lets me increase the MB FSB from 100->133 but then the system won't post. My knowledge doesnt go beyond that as far as tuning the bios.
What I'm looking for is someone who has this board running stable and can give me some bios setting that work. I'm not bothered at this stage to overclock, I just need it stable again.
At first I thought it was the onboard graphics becuase it always crashed while running opengl stuff for more than a few minutes. But I use now a seperate card for graphics.
Thanks to anyone that can help.
Peter