A stab in the dark here but I am wondering if your RAM is up to scratch and could you let us know just what you have in the way of RAM sticks in because 6GB makes me think you have a combo of different sized sticks and if so probably not matched to one another.
Personally I would be doing this RAM - Test with Memtest86+ < it has to be done for at least 8 passes unless errors come up straight away.

As I said a shot in the dark but at least it would eliminate the RAM as a cause and I would invest in a matched pair to 8GB or use one stick at 8GB.