I wouldn't hesitate to suspect the G.Skill RAM. I'm on my 2nd RMA with them. Works fine for a few weeks, then start getting random lockups and reboots. I'm not even overclocking the RAM, and in-fact am running them under spec (850MHz even though they're rated for 1GHz).

A little searching reveals that lots of people are having problems with their memory. Seemed like a good company at first, now I'm rather disenchanted with them and am wishing I got something else. I would've had to go with CAS5 instead of CAS4 though.