Exhaust overheating

I have a 1997 Grand Touring that I am having problems with. The first one is when starting out I sometimes have to feather the throttle in order to get enough RPM's to engage the clutch. It's almost like the engine is starving for fuel. Second once the clutch engages it runs fine under 40MPH. When you go faster than that end when putting a load on the engine like going a long distance uphill the exhaust box overheats to the point where the plastis gets so hot it smoke and gets soft. Yesterday it even caught fire... There is no fuel leak and the engine is not overheating it's strictly the exhaust box.

Exhaust overheating

Exhaust overheating

I have a 1997 ski-doo 583 Grand Touring. I runs great under 40 mph but at higher speeds and under a load (going up hill) the muffler box overheats to the point where the plastic gets so hot it melts...and recently even caught fire. Any ideas what would cause that? In addition, when you start to move forward it's like the engine is starved for fuel and you need to feather the throttle enough to engage the clutch. Any ides would be helpful.

Exhaust overheating