From what I remember, You should have at least 3 times the exit area as intake, so if you have a 2 inch intake, you should have a 6inch exit. It also sounds a little like you might be running it *lean*. 2 things you can try. First, pull the cowl and fly without it. If the problem goes away, you know it is an overheating problem. 2nd, richen it up. maybe 1 full turn out. Try that and see what happens. Let us know.
C2K
(edited word in stars)