Do you know what EPA is? It stands for end point ajustment. Your servo may be trying to move more than your wheels can actually go. Or your throttle servo may be trying to open up your carb more than it can actually go. When it does this your servo may strip gears, heat up, burn up or break however they break. Check your manual for the section on end point ajustments (EPAs). Set it so your servo only turns as much as the parts want to go. I can run a low quality JR servo racing hard for a LOT longer than 3 hours