Flying style could have something to do with it. If you fly with the thing at full throttle all the time, well, if it's marginal as far as power is concerned, that could cause either the motor or the ESC to burn up.

Take a look at the sticky at the beginning of the Beginners Forum that talks about prop chart and electric power and read down to a post that I put there that disucsses engine/esc selection for models. It may give you a good idea as to what you can put in the plane for nominal performance in the ranges specified.

CGr.