Mine appears to do that, too. Usually, when I'm using it, it'll work for a good amount of time (maybe 3-4 hours), then it'll start acting funny.

Once, when I had OS 9 on it, I got the dreaded error saying the cache was (or was going) bad.

If I leave it for a while, I can use it again for about 3-4 hours before it starts acting funky, so it *might* be a heat-related issue. Like you said, make sure all three screws on that heatsink plate cover thingy are fastened, and it is in fact touching the processor.