The overheat sounds a probable cause, make sure you have enough cooling on the engine.
One thing to check is, make sure the engine has smoke at half throttle, if it is not smoking then increase the pump pressure until you do, the engine is leaning too much and that causes the pre ignition that tends to throw the prop.
Running 30% nitro will make the engine run cooler as the nitro cause that, but you can use 20% no problem, but how much oil is in that 20% nitro fuel? it has to have no less than 20% oil, I run 30% nitro and 23% oil. Make sure you have appropriate fuel.

Try to provide more information to see if we can find what's going on.