Radeon 1300 Pro shuts off monnitor

I just installed a Radeon 1300 pro on my HP a250n (specs here: http://h10025.www1.hp.com/ewfrf/wc [...] &lang=en). Problems arose when I tried playing Sword of the Stars: Born of Blood (turn-based strategy). The game would load just fine but as soon as I started navigating around the starmap, the screen would go blank although my comp would still be on and game sounds were still present. The problem also arose when I left my computer on for a while, came back after about 4-5 hours and found that the monitor had shut off again. I'm almost positive it's a heat-related issue but also wanted to be sure that it's not my motherboard or the card itself.

I have upgraded to 1.5 gb of RAM, a 550 watt power supply, and an 18" EVGA Gateway FPD1810 flatscreen. I've tried to address the heat issue by running my machine with the side panel off and a box fan blowing directly onto the machine, but this doesn't seem to help. I also tried uninstalling and re-installing my drivers but to no avail. I've tried using both VGA and DVI cables but neither makes a difference. I downloaded the most recent drivers but those only seem to make the game shut the screen off faster.

I'm somewhat of a noob when it comes to computer hardware so any advice/help would be greatly appreciated. Thanks.

That Radeon card may be defective, but in any case is not a noteworthy performer. AGP options are limited, but check out Cleeve's article and thread, the best gaming video card for the money. Rather than spend more money on a better cooler (which may or may not fix the issue), it might be better just to buy a better card.