Actually, most games work okay. After playing through each of them, I now see that it's really only Oblivion that exhibits the problem. Now that I think of it, maybe Homeworld 2 had a bad frame rate from the beginning. Anyway, I tried uninstalling and then reinstalling Oblivion to see if a fresh start would solve the issue, but to no avail. And since all my characters relied on a ton of add-on content, I lost them all for nothing.
I did notice that the game runs normally with the resolution set to 640 x 480, but c'mon, give me a break. I had it running at 1024 x 768 at 40 - 50 fps before. Man, I'm just about ready to toss this computer in the trash.