you say that the valv cover doesnt get hot, the problem is that that isnt the part that get hot its the crankcase and the piston shroud
im using the same setup as you but in a different plane maybe a different sizqe tank, i belive mine has had some troubla along the same lines as yours as well have you adjusted the tappets and made sure that the valves are at near perfuect tdc as thay can