There are two contact forks on the backside of your trigger switch,one of them is trying to ground off on either the GB case or your selector plate.Flip the GB on to its right side and look at the forks.They come into contact with your selector plate,and both should be making equal contact with your selector plate,if not,adjust the culprit.If this isnt the case,take a look at your trigger switch,the trigger shuttle could be preengaging your trigger switch block,a tighter spring will solve this problem.