oops soz misread - thought it was feed issue dooohhh
yup trigger switch area, carbon build up maybe and trigger contacts getting stuck
(could be cut off but to keep firing on auto means the plunger is remaining closed)
it should stop firing on auto when trigger released
so it is getting wedged on the contacts for some reason and spring not pulling it back
(spring must be still there but maybe a bit weak or the contacts are pinching and grabbing the switch too tightly ?)
new switch may be needed, the old one might be ok to clean up but wise to have a new one in case it is shot to crap