Hi!
You are just simply running it to lean!
Just open up the highspeed needle and your engine will work OK.
No additional heatsink or heli head is required. Remember our small glow engines are getting the most cooling from the methanol in the fuel not by the air itself.They are so to speak semi liquid cooled. So don't run it too lean. And don't use more than 5%-10% nitro.
10x6 is to small prop...put on a 11x6 APC.
Regards!
Jan K
Sweden