Just got a bike on craigslist and as I'm going through and making it my own, I go to clean and lube up the chain. As I'm back pedaling though, I notice the chain going up and down. It'll start here _ and about half way through the rotation go here - (very crude example, sorry).
Hoping I didn't get a dud, but any suggestions? Potentially a chain issue? The bike is an older Fuji Touring bike (Saratoga).