sounds to me like a stiff link...put it on the rack and run the chain around. The stiff link will make the derailleur move back and forth when it goes through the pulleys. You can grab the chain on either side of the stiff link and flex it (sideways) to free the link, or you can put your chain on the second set of notches on the chain tool and give the stiff pivots a crank or two. Once it flexes okay you need some lube and you're in business, unless it has stretched beyond function (doesn't sound like that is the problem, tho).