Ouch;

Is the cable to your rear derailleur lubricated and in good working order? Mtb’s are known to pick up foreign material, water and other “crude” that greatly shortens the life of the rear derailleur cable, especially the short run at the derailleur end. Many have “grub” or “Nightcrawler” protectors to guard entry for this critical segment. If the cable doesn’t move freely I believe it can cause sluggish shifting. In which direction may depend on your derailleur spring design, is it conventional (homes to small sprocket) or Rapid Rise (homes to large sprocket)?

When did this problem begin? Has it just occurred recently? If so please check the angle of the derailleur hanger, it's exposed and vulnerable to damage from impact (dropping bike on right side). The derailleur jockey wheels should line up parallel to the sprockets, if they don’t it can cause all kinds of silliness in shifting at the rear.

See http://www.parktool.com/repair_help/DAG.shtml for illustrations of a bent derailleur hanger.

As for Adjustment of the derailluers, two excellent links are:

http://www.parktool.com/repair_help/FAQrindx.shtml

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

Please review these links to see if they are of merit to your issue. Thanks!