Looks like the problem is not the limit screws which control the lateral motion (so that the chain doesn't fall off on either side of the cassette). What you need to adjust in this case is the b-tension screw. The b-tension screw adjust how close the chain pulley is wrt largest cassette. Here is the link to sheldon's description:

http://sheldonbrown.com/derailer-adjustment.html

Here is one on Park Tool's website with more pictures. See section on b-screw adjustment..

http://www.parktool.com/blog/repair-...nts-derailleur