Ok, making a drum sander than I found he

http://www.rockslide.org/drum%20sander.html

The only difference is that I'm making it approx 24" wide... I've got
it all built, but having trouble truing the drum. I get an ocillation
effect when then drum hits the sand paper, and I get a bunch of flat
spots on the drum all the way around it. It's kind of like a
washboard effect on a dirt road.

Any advice for this?