Just got my bike back from the LBS. New chain and new cassette. Everything shifts great except the chain jumps off the middle chain ring when I put a lot of pressure on it like out of the saddle up a hill. Does not happen in the big or small chain ring.
The chain rings have 5,800 miles on them with the middle having probably 75% or more. I hardly ever use the small.
The front DR is not touching or pushing against the chain and this happens no matter what rear cog I am on.
My guess is the chain ring is worn and doesn't match the chain. Any other possibilities?
OBTW - Campy Veloce 10 speed. Rear cassette is original replacement 12 x 23 and chain is original Campy record Chain 10s