Have you oiled the freewheel mechanism? If not, then it's likely that the freewheel is at the end of it's useful lifespan. These Chinese derailleurs are, almost without exception, a cheap piece of cr@p. You should replace it with a decent (meaning SRAM, the only good manufacturer of freewheels left) new one, or else scour eBay for a passable used one. New, a 6-speed will set you back $35, and a 7 or 8 speed derailleur will cost you $40, both at Nashbar. There are often specials on these, too.