Why does my computer lag after being on games a while?

My computer can run games pretty well, but after being on any game for about on hour or so, it'll start to seriously lag. This isn't only the graphic intensive games like StarCraft II, but even much older games with much less graphic capability, like Flyff and Neverwinter Nights, the originals. I'll just play for an hour or so, it'll lag unbearably, and I'll get off, do something on the Internet for a half hour (which still runs fine, even right after the major lag), then get back on and the games will run perfectly. Anyone have any idea what's happening/what's causing this? I have a GeForce 8500 and a dual core processor, still over half my hard drive space left, and 3GB of DDR2 RAM.