well all i can say is
check and double check everything
check the sear and bolt lip for wear
make sure the on/off pin isn't bent and moves freely
then i'd say install your stock on/off just to make certain the ult on/off is the problem
oh ya make sure you have enough air in your tank
i dont know how many times my gun has started acting funny and worried me for a second just to see i only has 500 psi in my tank