Chain jumping on rear cassette

kenan Posts: 952
edited April 2011 in MTB workshop & tech
Building a bike up with parts from another frame. Have fitted a new chain with older cassette and chain set. When on the bike stand the gears work fine, however when under load the chain is jumping, feels like it is jumping forwarded not up/down a gear. I assume this means I need a new cassette, is this the case or is there another solution?