Well correct me if I'm wrong but here is what I see.
You have a computer that is about 4~5 years old. You upgraded the video card. Now your video card cuts out after playing for a bit?
Your power supply is just too old, from the 400w starting max load and factor in capacitor aging and the amount of hours on it... i would say that there is a high likely hood of your power supply just not being able to supply the power necessary.