We have a projector we watch movies with in the basement. It started acting up (shutting off picture after 40/30/20 minutes and then less and less time, fan still running, had to be unplugged to stop it). DH is a computer guy, but not an A/V guy, per se, so we called the manufacturer and they referred us to a local shop. After (they say) running it for 8 hours a day for three days they could not replicate our problem. They got to keep the $40 look at fee. With nothing to lose, we took it apart. All the circuitry looked fine, so after we removed some internal screws, we found a fine mesh screen all caked up with dust in the entrance of the fan baffle (?), which we removed and washed, and didn't put back in and did the same with the outgoing screen, but put it back in.
Long story short, it was a heat problem, we solved it, and we get who knows how much more use out of our 6 year old projector. Too bad about the $40.