More suggestions:

This engine needs to run cool. Try using smaller props and low nitro fuel: 5% or even 0%.

Some guys say their 91s run great. The only thing I can deduce is that their engines run cooler for some reason. Of course, you would always want to run this engine with the carb set as rich as possible.

This engine may be just a bad design: too much displacement in too small of an engine. I switched from a 61 to the 91. It produced more power, but not nearly as much more as I expected with a 50% increase in displacement. The 91 also really likes to drink fuel. I would look for OS to do something about this engine, like a new version with bigger head, diferent carb, bigger frame and crank, etc. The 1.08 has been a great engine, but just very large and heavy.