[TokyoSeven]

As in it still has that little hook dealy that stock TM springs come with?
Did you reinstall it with the unique piston head attachment that holds the piston head onto the piston but also holds spring in properly as well?

I would have to say it was the gears and the piston combo, but you say that its suppose to work with regular pistons.

Whats the motor?

I dont know how or why but Im seeing this image in my head. Since there is a weaker spring, Im seeing the gears driving the piston back with alot of force but returning to their standard position before the piston can be pushed back into the cylinder by the spring, so the piston trys to come back forward but is caught on the gears.