I've only read about guns going slow, not fast!
Hmm how long have you had the machine for, is it a problem that has just started?
The only thing I can think of (Silverball taught me this)...Is the game an export (ie European). If you can test voltages at the coils and compare this should tell us?
I'm thinking if the game has not been jumpered correctly for AUS 240v (assuming you're in AUS I didn't check) there might be too much juice pumping to the coils.
