there is a lever inside the mechbox, you can see it from outside of the mechbox... it's designed to throw the trigger connection out and break the circut when in semi auto, this may be related to your problem, on one end it runs to where the gears are i can't remember exactly but there is a egg shape on the gear that moves the lever... come to think of it this probably isn't your problem but it doesn't hurt to check.
my p90 is warn out and the gear timing landed perfectly so that lever just stopped my trigger all together and i couldn't fire at all, i just unscrewed the motor and took it out then spun the gears back to nutral. gun worked again. hasn't happened again yet..
maybe that lever is broken?
__________________
.
|