|Your problem isn't gone; what is causing it is not being triggered right now.|
You have a fast processor; 256 MB RAM is a little low, but that shouldn't cause the probs you describe. I can't see that upgrading anything is needed at this point.
Try cleaning out the case: pop the cover, take it out on the rear porch and reverse a vacuum cleaner to GENTLY blow out the dust bunnies, dingleberries and whooosits that collect over time. Use a soft, clean brush to clean between the CPU heatsink fins and the vent fan blades. Check the rear of the power supply and case vents for dust buildup.
When done, make sure that you don't jiggle or dismount anything like the memory sticks, video card (if you have one), etc.
If it does it again, repost and we will get really serious.
Wish you the best!
Give a man a fish and you feed him for a day;
Teach a man to fish and you feed him for a lifetime;
Then industry pollutes the water and kills all the fish.