you need to adjust your deralleur tension screw.its the one that sits against your hanger.put the bike in first gear,look at how close the 2 deralleur wheels are to the cassette.start with the screw all the way out.this should make the wheels chatter on the cassette.turn the screw in 1 turn at a time,when it barely stops chattering,your good to go.now adjust your deralleur the way you normally would.this should fix the gear jump at the last gear.