Try re-shim the gears then close and tighten up the box with only the gears inside. If you can spin the gears with your pinky then your golden. Use your own judgment and adjust accordingly. I personally would never use an m120 spring. IMO It is way to much strain on everything. I have only had trouble with heavy springs. Gears and pistons wearing down. I would spend the extra money and go efficiant. Go with a pdi 130% or 140% spring and make up the extra fps with extra internals. My g36 had a 140% spring in it and shot at 400 FPS everytime with these simple
internals and not to mention your ROF will increase majorly.
As for your problem I would agree with everyone on #1 taking the tin foil out and use a proper fuse. Next try the shimming. And then if worst comes to worst its the motor. Or hell maybe its just cursed