I mentioned this in the What Game are you Playing thread, but I felt that if I really did want responses I should make a separate thread.
Basically, up until this week I'd never had any problems gaming on my pc. Granted, I don't play much (Portal, Civ V, and some random old games) but Portal had run beautifully and Civ V was okay on medium settings. I tried playing Portal 1 on Monday, and after about 3 minutes of gameplay the frame rate totally dropped off, from 60 to maybe 20 or less. Like I said, I've never had this issue before, but it made the game unplayable.
The exact same thing is happening with Portal 2. I'll boot it up and for a couple minutes everything is hunkey-dorey, but then all of a sudden: big slow down. To fix it I would quit the game and reopen it about 10 minutes later (any less time and the frame rate was still bad) but eventually I stopped doing that and just worked with the slowness. It isn't always awful, but there's still no reason I can think of that I should have to deal with this.
Some quick specs on my pc:
-6 GB DDR2 RAM
-Intel DC E5200 processor (2.5 GHz each)
-GeForce 9200
-640GB 7200rpm HD
-1360x768 res
This machine isn't even 2 years old, and I really don't know what to do to fix the problem...

