Hi Murphyman,
I posted some time ago about the cost compromises HT receiver manufacturers make in order to include lots of features and underprice the competition: case in point--the Onkyo cooling fan. Instead of extensive heat sinking (costly), to cool the output transistors, Onkyo has been using cooling fans in recent years. The Onkyo I had ran very hot--far hotter than other receivers I've reviewed (Pioneer, Yamaha, Denon, etc.), and its fan never kicked in either. Be sure the 797 is really well ventilated, and if the warranty is still on it, have a service center check the fan's thermostatic control. Put your hand on top when you've been running it for an hour or so. Warm is fine; hot is not.
The impedance curve of your other speakers may have been quite different than the M80's curve. It depends on what frequency the curve dips to 4 ohms. But I suspect what's happening is that you are pushing the 797 too hard (big room), the output transistors heat up, and the 797's protection circuitry kicks in and shuts it down. Many of these receivers don't deliver anything near what they claim when you run them in the multichannel mode. So go easy or get a bigger power amp with a proper output section that's rated for 4-ohm loads, one with extensive heat sinking (it will be heavy).
Regards,
