Yes, heat build up is probably the number one thing that will shorten the life of your electronics, but unless it gets VERY hot in their for extended periods, that probably isn't the direct cause (likely a factor).
I'd say it's just a bit of age and a component in the receiver that was marginal from the start, but worked until now.
Sounds like a replacement is in order, because repairing this receiver, unless you can do it yourself, will probably cost about the same as a newer, better one. Take a look at the new Onkyos.
Cambridge Soundworks has some b-stock units for $273 right now:
http://www.bestspeakerdeals.com/stor...501&type=store