Originally Posted by Crankycrank

There's a chance your front derailleur just needs an adjustment since it could be the chain rubbing on it when you're putting down more power as the frame/cranks can flex slightly. Shift the derailleur all the way to the right and see if you can push the derailleur a little further to the right with your hands. If so, then just tighten the cable adjustment slightly until the rubbing goes away. Also, could just need an outer limit screw loosening but not likely as it was working before and usually don't go out of adjustment but not impossible.