I know this sounds really basic but could it simply be out of adjustment. You need to adjust the screw that tells the derailleur where to stop properly or it overshifts and drops the chain. This is how I stop the problem. I bought a compact derailleur, but then it wasn't good for my eliiptical chain rings so I took it off and got another one that worked better for my crank system. If you are interested in buying it, its an IRD compact double derailleur. Its for a braze on but I also have it attached to a 28.6 problem solvers clamp. PM me if you are interested, it looks brand new as it has ~50 miles on it.