Hi

My bike has an annoying habit of changing to a smaller rear cog as I am riding up hills. This only happened since I had the rear cogs replaced due to being totally worn. My old LBS replaced it with a Sun setup which was fine at first but then started to slip, I moved house, took it to my new LBS, they said the cassette was moving and replaced it with a modern looking (ugly) Shimano number which was fine at first but is doing the same now - any suggestions?

(60s lightweight - Campagnolo Nuovo Record mech elsewhere ).


Thanks
Mark