Could just be your contacts are dirty. I had the same problem with an M733, had intermittant voltage/continuity in semi mode, but nothing in full auto. After a few attempts at fixing the problem without opening the mechbox, I said fuck it and ripped the mechbox open and got access to all the contact points. A few Q-tips loaded with lighter fluid later, I put it all back together and had it firing 100% in both modes.
Contacts need cleaning once in awhile, if corrosion or grease happen to build up on the contacts, the gun will stop doing anything when you pull the trigger, making it seem like a broken fuse or a dead battery.
BTW, Greylocks taught me this a couple years ago, figured I'd mention this because he's not just a "crusty, old, flaming chairsofter who doesn't play anymore", he's got a fair bit of knowledge on inner workings of mechboxes. Not that he let's on anymore.
