Have you tried removing all the compression parts piston, spring, spring guide, etc. and just running it with gears only? If it works outside the gearbox, works inside with no load, but doesn't work with a spring and piston, your motor is just a dud. I had similar problems with some old CA and JG motors that were fried.
I say either get a replacement or install a new one if it continues to give you trouble. No sense wasting all your time and effort into one stupid little motor when you can get a replacement for so cheap. Also get rid of those tamiya connectors. The deans (and even the knock-off t-plug) connectors are a million times better in every way providing you solder them on correctly. The overall size isn't what matters, there is just so much more surface area on those deans connectors of good metal vs. those crappy tin tamiya connectors that fall out or burn up.