Yup, I've got an interesting issue right now. with my wife and I the chain jumps on the middle chainring when under high load (like jumping across an intersection after the light first turns green). The chain literally lifts off the ring causing us to flail a tooth or two until it recatches (still on the middle ring).

I've measured chain stretch and the chain looks just like new. The chainring doesn't look worn at all, but the chain still jumps.

I can't reproduce it with just my peddling force, it's got to be both of us.

I'm going to buy a new chainring and see if that helps.

Sheldon