I have a Campy Nuovo Record rear derailleur from the mid-1970's. Recently, my old six-speed cassette stripped and had to be replaced. My mechanic put a narrow seven-speed cassette on it. He also replaced the chain. It all works fine until I have to stand on the pedals, such as going up a steep hill in first or second gear (the cassette ratio is 13-23), at which point the chain slips down to the next smaller ring. I've tried fine-tuning the derailleur, with no luck. Is this a problem of component compatibility? Any advice? Thanks!