The bike is a 2002 Cervelo P2K (Ultegra). No problems stock. I bought a pair of Zipp 404's, with a 12-25 casette (stock casette is 11-23). When I put the Zipp's on, re-adjust the hi and lo limits and tension on the rear deraileur, everything works fine -- in the garage and on the flats.
The problem is during heavy loads, i.e. climbing, during which the chain drops off (to the inside) of the front (small) chainring. Further, it only happens when in the two biggest cogs. Front deraileur is not causing the problem, I've triple checked. I've had two separate bike shop mechanics check and tweak it, and it works fine on the rack and in the parking lot, but get on a hill and chain drops. I've been able to replicate the problem on the trainer, but unfortunately, the LBS's are closed today Sunday. Any suggestions on what to check? Really would like to take advantage of the nice weather to get a ride in.
Many thanks in advance for any direction you can provide!