I've got the 120AX and ran both a 15x8 and 16x8, no problems. I would check the fuel line - are you getting sufficient fuel when pulling to vertical? Maybe not enough fuel and therefore not enough oil/cooling? I use 10% nitro, and after a flight of tons of inverted, I can grasp my engine without any discomfort - runs very cool.

There is also a thread on using this engine with a Pitts muffler, and the poster talks about crimping one of the exhaust vents closed to gain more pressure in the fuel tank. Personally I use stock muffler with a perry pump - try a pump, may solve all your problems.