Keep losing video signal

I installed an XFX GeForce5200 256MB vid card in a Gateway with a 950MHz processor, 250W power supply, and 128MB of memory. Ran fine for about 1 week, then I started getting video blackouts after computer runs for anywhere from 20 min to 1 hr. Computer stays on, but monitor goes into standby. Since then, I’ve installed extra case fan, 350W power supply, flashed bios, experimented with the AGP aperture, upgraded AGP and vid drivers, and added 128MB more memory. Returning to old video card (Gateway proprietary Nvidia Vanta TNT2 w/16MB) solves problem, but family needs more video memory to run new games. Please help!