I'd go with Crash here. Probably RAM related. First thing I would do is take the covers off, remove the RAM, get a can of compressed air and blow all the crap out of the machine. You'll be amazed how much will come out. Just don't do it in the house! Then repeat the task with the RAM.
A colleague at work had something similar, and the local PC repair shop wanted £150 to fix it without even looking at it. Took me 5 minutes of my time, and that person saved a lot of money.
<font color=blue>"Some people believe football is a matter of life and death. I'm very disappointed with that attitude. I can assure you it is much, much more important than that" - Bill Shankly</font color=blue>