I find when they fire as you air up it's usually due to a loose fitting oring on the ram, or the bolt is half way between fully back and fully forward or the noid is a bit dry and sticky. When I first got my etek used, it hadn't been fired for a long time. Once I gave it a new ram oring and some oil, then used it for a day the issue went away. Worth a shot.
ETEK 3LT, with 14" critical .680 barrel and cure 3 + bolt F/S