I have found that if you have a sticky or stiff link as detected in an earlier post by backpedaling and watching the chain run through the rear derailleur. It can be fixed without any tool by locating that link centrally between bb and rear axle at the bottom. Put on gloves as this is dirty work, simply grab the chain on each side of the stiff link and give it a good bend side to side. I mean attempt to bend it sideways ( counter to the way it normally bends ) back and forth with some moderate force. I do not mean bend it the normal way the chain flexes but against the grain. After a few of these bends give it a try and I think you problem will be solved. It has worked every time for me and can be done on the road without tools.
Ray Homiski
Elizabeth, NJ