Those are the adjusters. There should be 4, one for each shoe. They are an eccentric cam that rotates and pushes the shoe outward towards the drum. The jumping must be when the highest spot on the cam goes by where it pushes the shoe. You have to turn all 4 adjusters so they push the shoes out close to the drum. If the adjusters rotate all the way around the shoes may be worn out. I would pull the drums and take a look in there.