One of two monitors going blank after 30 minutes

I recently purchased a Dell XPS MT 435 against my better judgment to save a few bucks compared to building my own. It has a Radeon HD 4670 (512MB) with dual DVI out. As rated, it appears the power supply is too weak for the card, but Dell of course says it has been tested and is fine. It ran fine with a Dell 20 inch monitor at 1680x1050, but I recently purchased a Samsung 23 inch monitor which runs at 2048x1152.

I hooked the Samsung up along with the Dell and it was fine for about 30 minutes when the Samsung just went blank. The Dell was still going just fine, as was the computer. This is without doing any gaming at all. I even tested it with just letting it sit there and go to the extremely basic windows screen saver. Restarting just led to the same problem after 30-45 minutes. I switched the DVI cables but that did not help either. I also noticed that even moving one of the monitors caused them both to flicker on and off quite a bit (like they do when there is an auto-adjust going on).

I am thinking it was one or three things: overheating, too weak of a power supply, bum card. Thoughts? Thanks.