Someone on GeekSpeak suggested overheating when I had a weird problem, and that was it. As a matter of fact, that's been the problem virtually all the time when strange things have happened.

Now I take off the fan, and suck what I can off with the hose from a vacuum cleaner. Then I reverse it and blow more stuff off.

Then I take an aerosol can of "Electronic Cleaner" liquid and spray it on the CPU heat sink. I turn it sideways and catch the resulting goo with a paper towel.

Then I spray it with compressed air from an aerosol can. Has always worked.

Don't know if that's your problem, but it's been an easy and cheap solution for me.

Arf