I had the same problem too, your trigger settings are too close to the actuator, the vibrations is setting off the trigger, making it full auto
Straight from wiki:
PSP Ramping – Ramping begins after 3 shots; the player must maintain at least one pull per second to achieve/maintain ramping. The marker may then fire up to (and no more than) three balls per trigger pull in a "burst" fashion. Rate of fire cannot exceed 12.5 balls per second (as of 2011), even if the player pulls the trigger 5 times per second or faster.
Millennium Ramping – Ramping begins after six trigger pulls at a minimum rate of 7.5 pulls per second; the player must maintain 7.5 trigger pulls per second to maintain ramping. Rate of fire cannot exceed 10.5 balls per second. When the player ceases to pull the trigger during ramping, no more than one extra ball may be fired after the last pull.