To me it sounds like the engine is running hot because its too rich. What is happening is the engine wants to push the truggy and its being held back by the load of the truggy, then you are adding to the load by dumping in more fuel than it can burn. But when you stop that added fuel cools the engine down to 210.

You are about 1/2 a gallon of fuel right now, so the temps are not extremely hot for that amount of metal pinch. I would say lean the top so the engine can clear out at RPM. Then go a head an richen the LSN to bring the temps down (if they go up). Make sure your idle gap is set to 1 - 1.5mm.

The 20% Nitro shouldn't matter.