I have a problem I can't seem to remedy for my road bike. When I'm in the middle chainring and the 2 lowest cogs on the cassette the chain wants to fall down to the small ring. If i keep pedaling it will climb back and then drop again. When I first bought the bike it never happened and then gradually it started. The front derailleur angle is good and the rear shifts great, and it only happens in the middle chainring. I can granny the big ring and all is well. I was thinking chainline, but since it was fine out of the shop and it gradually started makes me want to think otherwise. Any ideas. It's a RPM crank (never heard of it) and ultegra rear,105 front and a sram chain\cassette. Thanks