I have flown mine with the RAM and the ram pipe with an open system with no problems. You may not be getting enough air around the outside pipe. You may need a small inlet along the bottom to let air in if the plane is sealed tight. The new one I have, I cut out the slats on the side and bottom for air with the closed system.
If it still stays warm, paint it with BVM heat retardant
Any chance of bringing it to Michigan Jets so I can see how you finished it.
Burt