A few suggestions, some already pointed out.
It may be the graphics card conking out (had a similar thing happen once to an NVidia pci-X card, and it did exactly the same thing), or the RAM failing. BUT, it could just be either of them not getting adequate cooling, which has several remedies. First make sure all the fans are keeping going, second, clear all the dust off any fans, vents and heat sinks so that the air can circulate, third, make sure that everything is seated properly so that things are not conking out when they warm up and shift position due to expansion.
It could be a software glitch too, with the drivers having corrupted in some way, you could try a few remedies for that. Doing a system restore back to a point when you were not experiencing problems (back up anything vital to a safe place before doing that, as a precaution), reinstalling the graphics card software is another option, as is going to an earlier more stable version of it. A good swing at things with anti-virus and malicious software removal tools probably wouldn't hurt either.
Another somewhat obvious thing to try, is to reinstall SH3, the current installation might just be screwed and have lost some vital file or other. You can back up your save games if you want to keep a campaign going, but try it in default format after a re-install first, to check stability.
Chock