Sounds like it was probably a power or heat problem. When you replaced the card, did you notice that there was a lot of dust inside? You can get a can of compressed gas and solvent for cleaning computers (or get one from a camera store which just uses a squeeze bulb), and that can really help. If you've upgraded your machine, you might get an additional fan. (I was having the same problems you were, and eliminated them by aggressive cleaning and an auxilary fan.)