Originally Posted by
Ablaut
I have a similar problem with a late 80s Campy cassette and new chain. It seems the slip only occurs when I'm in the three lowest gears. During normal spinning it's fine, but if I'm accelerating or up out of the saddle, it'll happen repeatedly. The cassette doesn't appear to be worn out. The clunk-clunks and loss of acceleration is driving me crazy... I'm at a loss about what to do.
A worn casette with a new chain will skip like mad when youre mashing, particularly idf you're doing it on one of the more worn cogs.
Here is Sheldon Brown's take on it (scroll down to where it says "A new chain on a worn sprocket").