I'm having issues getting into the largest sprocket(28t) on the rear whilst using the smallest front ring (triple). All the others are going in fine, it's a new cable, a new casette and chain. The rear derailleur is an old but virtually unused long cage Exage 500 LX. The shifters all work fine, basically it goes on the sprocket but keeps jumping. I should add its a 7 speed casette on an 8 speed hub with appropriate spacer. Any ideas?