Hi. I went through the same thing just recently. My chain (CN-HG53) was jumping, and I have fallen because of this by loosing my balance @ 18 MPh in the woods. It was nasty, but anyways check from the back side if the front chain ring, cassette & both pulley's on the derailleur are in the same plane. When you hit the derailleur it usually bends, it might be the dropout only, but in worst case it's the derailleur. Check every link of your chain for straightness. Maybe it's twisted. Hope it helps. A.