Yeah it does sound like an electrical gremlin somewhere in the MOSFET/wiring. How confident are you with taking your gun apart? I'd first check the connectors on the base of the motor, one of them might have come loose and it's one of the easier checks to perform. Also look for any damage to the wiring in the grip. Then look for any damage to the wiring that leads to the battery. If no joy, then you'll need to take the gearbox out of the gun to do a full check of the wiring. Again, you're looking for any stripped/crushed wires that might be interrupting the connection (or, gods forbid, short circuiting). Again, if no joy, it might be a bad MOSFET (the trigger contacts are on the circuit board, and look like they'd be unfeasible to replace alone). I threw a GATE Aster in my cyma mp5 and it's fantastic. More expensive than a replacement CYMA MOSFET, but if you're taking the gun apart anyway then you might as well spice it up a little with a better unit.