The problem with the rotten dog sys11C mpu / driver board is a design fault.
they have 2 of the inputs for the special solenoids arse about (one of the pop bumpers and the right slingshot)

in some earlier system11 games the sling shots were also controlled via a direct switch input but on sys11c (and some sys11b games) they are triggered via the switch matrix but the direct input is still usable on sys11c.
so when you triggered the sling shot by the sling shot switch it is pulsed and when you trigger the pop bumper it also fires the sling shot but from another input on the board 

but its all sorted. I will post a photo soon.