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First, make sure the cam, or eccentric as Joe calls it, is not stuck. It should slide back and forth in the "cam shaft." Consult the Red Book if this is not clear.
But I highly suspect the drum is also out of round. It is most likely a combination of a high spot on the shoes and an out of round drum. Check the shoes first like Bob suggested or glue some course sandpaper to the inside of the drum to sand away any high spots on the shoe linings.
As far as the out of round drum is concerned, the only course of action is to take it somewhere to be turned. If it is the original stamped drum, throw it away and get a new cast iron drum and hub and have that turned.
I don't know where you are located, but there are some good shops in the country to take your drums to. Let us know where you are located and someone on the forum can suggest a good place to work with.