I'm not a computer guy but when I used my combat flight simulator 3 for a while with 512mb of RAM it would jump and stutter. I insatalled 1024mb of ram and operates much better for a longer time but still will load the ram and require a computer restart.
The problem was not there if I used minimum resolution.
You may have enough RAM as 256 MB is only about 190 MB usable.
Just a thought.