time to run some diagnostics:
run memtest:
http://www.versiontracker.com/dyn/moreinfo/macosx/17156
Ideally, run it from Terminal, running 2 copies, each testing half the memory (to stress both processors). If you don't know how to do that, d/l and run Rember (if you make a copy of it, you can run both copies at once).
Rember is built on top of memtest.
memtest is a much tougher memory test than anything else (esp. better than Apple's hardware test).