ORIGINAL: jwmm
I had a similar problem, found the cause to be over heating during flight.
I just increased the size of the air intake on the cowl.
Engine was sweet after this.
John
Caution on opening up only the air inlet to your engine.
The air OUTLET from from your cowl should be roughly twice the inlet area.
This should have the effect of creating a negative pressure at your cowl outlet.
Otherwise you stand a chance of damming up the hot air inside your cowl.