I have this 6500 mile, ten year old Mongoose hybrid with twist grip shift whose RD is no longer shifting correctly. After 4000 miles or so it would only shift smoothly one way, larger cog to a smaller. I went thru the adjustment routine in ParkTools and got it to shift smoothly the other way, smaller cog to larger. But never smoothly in both directions. Right now it takes two 'clicks' on the index to shift to a smaller cog, but the normal one click to a larger. Cable and twist grip work fine. It does not look bent. A replacement RD would be about $25; but I would like to fix it, (Shimano C105 7 speed freewheel Megarange, low normal).