I am running an inverted OS91FX with a Slimline inverted pitts muffler. On the first flight, the engine ran fine on the ground, but after a couple of minutes it lost power and wasn't running well. Second flight, same thing. Cleaned all the lines and carb and put the engine a stand and it ran great. Back in the plane, same result. Decided to fly once with the cowl off. It ran well the entire flight. Cowl back on, it ran ok for a couple of minutes then loss of power, etc. I believe I have an overheating problem. Anyone running a similar combination have an overheating problem? The in-cowl pitts muffler doesn't help the situation. Thanks.