I have a suntour mountech RD on my 1983 Miyata 610. It keeps throwing the chain off the lowest gear and it gets caught in between my freewheel (5 speed 13-32) and the plastic disc on the wheel. I have suntour friction bar-end shifters. I have had this issue looked at by two bike stores, and they will charge me $10 to do some quick simple tightening of a piece of the derailleur and it will work fine... for about 2 weeks and then it starts happening again. I feel like if I just understood the mechanics of it even a little bit I could fix the issue, but I feel like at this point I'll just have to buy a new derailleur? It's a shame because otherwise the shifting is flawlessly smooth. Any advice?