The exhaust area for the air going into the cowl needs to be 2-3 times the inlet openings - flying without the cowl as SeamusG suggests would rule out insufficient cooling if the problem still exists without the cowl on. Also when tuning after coming back to max power on your needle adjustment you still need to richen the high end needle about 3 clicks, as the engine unloads in flight.

The exhaust area [for the cooling air] needs to be at the back of the cowl usually down by the muffler exit.