Originally Posted by
Bizman
I did notice one time when this problem happened if I would pedal "backword" the chain would become very slack on the top dropping down and touch the chain stay. Of course after I rode it the problem would quickly go away and you could pedal "backword" with no problem being slack. Another time it happened and I spun the rear wheel the complete drivetrain would spin (rear wheel, chain, cassette, & pedals).
That's a sign of a gummed-up ratchet mechanism in the freewheel. Remove the freewheel from the hub, soak it in solvent until it loosens up, then run some oil into the mechanism by dribbling it in through the gap between the inner and outer freewheel body. Be sure to use a mineral based oil, as vegetable based oils degrade into the gummy varnishes that cause this problem.