ok i have a 98 custom
lightweight rear bolt, aliminium powertube, upgraded front bolt and new valve and all orings etc. standard single trigger

have recently bought a response trigger and installed it fine, dry fireing i have it tuned in shooting pretty near the 15bps (well sounds about like it) and holds in sweet spot no probs keeps firing until the bottle runs out of gas

however when firing balls will only shoot maybe 3-4 balls then loose its sweet spot.

ive been through maybe 500 rounds playing with it and even tried the pen spring mod aswell but still wont stay on the sweet spot with balls in it

any ideas as to what could be wrong? or will it just be me unable to keep on sweet spot with balls in it?

have searched google etc with very little help

cheers