i am assuming this is an automatic?

i think what you are feeling may be the torque convertor. I don't own an automatic but torque convertors are rated to grab at a certain rpm range. i imagine when you pass under that rpm range the torque convertor lets go, the freed up drag of the transmission allows the car to rev up again. until the computer catches up and continues to cut fuel.

again this is all speculation but it used to happen in an old vw bus i had, and also in a friends older nova.

does this happen more than once on the same coasting?