I am helping a friend with his Ritchey Break Away steel bike equipped with full Dura-Ace 7800 group (except for FSA compact crank, 50/34) and a 28 tooth largest cog. After returning from a trip with the bike (frame) broken down into its two parts in its travel bag, he reassembled it and attempted to ride it. With the chain on the big ring up front, the drailleur and chain were skipping ONLY ON THE FOUR LARGEST REAR COGS. There was "perfect" adjustment and no skipping on any rear cog when using the small front ring. This defect cannot be adjusted out. Upon close examination what I see is that when the chain is crossing at a sharp angle from the big front ring to one of the large rear cogs, the chain gradually slides to the right on the front-most (upper) jockey wheel until it runs into the outside (right most) plate on the derailleur arm. When it touches the plate, it jumps back to center itself on the jockey wheel, and the process starts all over again. This does not happen when back-pedalling when all is smooth. The larger the cog (further left the derailleur), the more frequent and more severe the skip. The drive train functions hardly at all on the two largest cogs with the large front ring the chain is so bound up. I checked the jockey wheel teeth, and they appear to be in good conditon with minimal wear. Besides this issue with the large front ring and largest four rear cogs there are no other problems whatsoever.
Not to skew anyone's response, but let me just say that the travel case is semi-soft sided and some crushing could have occurred. I can't see any misalignment of the derailleur hanger, which by the way, is very substantial and made from steel. The rear triangle was always protected with a dropout separator to keep the stays from being crushed. Has anyone encountered such behavior before? Any known causes? Cures? It would be easy enough to replace the derailleur in hopes of a quick fix, but we would like to avoid that if possible.
Any help would be much appreciated.