You can pull the side cover off fairly easily, and look for anything like a broken gear tooth or piece of grit. About half way down this page: http://www.flickr.com/photos/joeharper/3872936498/ Joe Harper explains how to remove the side cover from an A12 (note: a non-A cover comes off in a different manner), starting with "The screws to remove the cover are underneath...". You must be careful not to gouge out the screw head slots, but other than that you won't hurt anything opening it up. However, sending it in for repair may be the more sensible way to go here. If it is something worn or broken, you probably won't be able to fix it yourself, at least not without a parts back. But if you see something obviously out of whack, you may decide to buy a different back. I don't know what KEH gets for an A12, but they do have a guarantee. Or you can probably get a good replacement off eBay, if you shop carefully. Keep in mind any back you get (or the one you have) will need replacement light seals sooner or later. If you have your back repaired, new seals no doubt will be included.
Sometimes you can learn a lot by loading an empty backing paper,and cycling it through the camera,marking each frame with a pencil. There is likely a burr or somesuch on one of the gears.
Are you certain the insert and magazine are matched? (serial no's). They are matched for calibration at the factory,and are not normally interchangeable.
You may also try the empty magazine on the camera and testing the combination that way. The counter still works and you don't have to take anything apart. If it still stops, you can take the side off and watch it as you check it on the camera. There could be a loose bit wandering around it there.
FWIW both gears of the advance start and stop at the same point in their travel. They make one complete cycle, so it could be in the body too.