Weird engine rpm drop when coasting

Before I take it in and have the dealer guess on what the issue is I
thought I'd post and see if anyone has past knowledge of this issue.
2008 Aveo5 AT, 5k miles. When I coast with my foot off the throttle I
get a regular pulsing drop in RPM's. Normally from 50mph to around
35mph as the car slows the rpm will drop from 1600 to 900 or so and
then back up 1600. I first I thought it was downshifting as the car
slowed by I know I don't have 10 gears. It feels like someone is
touching the brake every second or two. I have tried this coasting
in neutral and of course I don't feel the slow down like when in gear
but I do the the rpm gauge bouncing up and down.
I have not have any codes or warning lights. I do have a scangauge
attached.
Any ideas ?
Steven