Without having the opportunity to look at your bike I suspect you have a sticky freewheel or freehub body rather than a derailleur issue.
Take the rear wheel off of your bike and try to spin the cogs backwards with your fingers. If you feel a significant amount of resistance, that's your problem. Set your wheel horizontally over a trash receptacle. Look for the seam where the fixed and moving part of the freewheel meet and spray a little WD40 in there. When the cogs spin freely, it's fixed.
Last edited by Retro Grouch; 10-18-09 at 06:57 PM.