You most likely have a cracked ring gear. I have experienced the same problem with my HAM-III, getting stuck in one position until you rock it out. The ring gear is the large gear about 5 or so inches in diameter, internally toothed, that fits just inside the bottom of the bell shaped upper housing. It is inexpensive cast aluminum (unless you buy the more expensive machined steel version). I think the aluminum version is about $10 - $15 or so. Easy to replace as long as you take care in the dis-assembly and re-assembly process and don't loose any of the ball bearings.
Mike NA9Q