Drum Sander Chatter

I just bought a Performax 16-32 drum sander. I tried sanding a piece about 20 inches wide, so I had to overlap. It seemed to be working OK, but when I got down to the last grit (80,120,150) I noticed some 'chatter' and some overlap which hadn't appeared on the coarser grits. Any ideas where the novice went wrong?
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Just a guess, You may be sanding too fast ot trying to sand too deeply for the fine grit paper.
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