Nitro has its own slew of issues, so let's not go there.

How hot was the ESC, and motor after each of these runs? Is it possible your gear mesh is too tight? Or perhaps you can gear down to a smaller pinion gear and try that?

Have you tried a different battery? Try a 7 volt battery, lipo or Nimh and see what happens? Is there a short in your wiring anywhere along the line? Is there water being splashed around? It can sit in the ESC where it is tight and enclosed so it can take days to dry off completely (unless you dry it out yourself with a hair dryer and/or if you are comfortable enough opening it up).