More Nitro in your fuel actually keeps it cooler dude.
Thats if you have it tuned right. Ever spilt Nitro on ur hands?
Its freezing cold. The only other reason i can think of is a problem we have encountered. The engine may be under stress because somewhere on the internal is not lubricated so its metal on metal and its ceasing up and getting hot.
Usually its the crank pin to the bottom of the conrod, pull it down and have a look, if the crank pin is PURPLE its ceasing up and its getting hot. Take it off and put some oil on it of some sort ( Caster , Klotz ) and well lubricate it.
Other than that i cant think of anything else making ur engine 300 Degrees!