Here's another vote for taking it apart and ridding it of dust, along with replacing the old, dried out thermal compound on the heatsink.
Yea, one is always surprised by the amount of crud that can accumulate around the HSF, plus thermal compound breaks down over time and needs refreshing. I used to work for a local audio sales/repair shop, one of our services was the local disco's (or any club that used high-powered amps) we would take the unit in, blow out any dirt/dust and remove and re-grease all the output stage transistors. These were usually bolted to a large heat-sink in a socket-type arrangement so they were relatively easy to pull and re-grease. We would remove the old, dry, thermal compound first the apply fresh. For all the customers we had, very few ever had an amp fail and these units were pushed at close to full output for 8-10 hrs/day.
