Originally Posted by
Drew Eckhardt
A wornout ring or bent tooth can do that. Fix it before it happens when you're standing - I crashed twice that way.
Chainline alone doesn't cause that, even using the big ring on a triple crank with 40.6 cm chain stays.
I once had exactly the same problem the OP described on an Ultegra triple equipped bike, and it was a worn middle chainring that was the culprit.
So, check that none of your chainrings are worn.