I occasionally get the impression the cruise control on my 2000 Chrysler Concorde LXi isn't operating properly. When set at about 60 mph, it occasionally feels like it down shifts. The RPMs rise to about 2900 to 3000 and I feel a shift in the transmission. The hill involved is from a series of "washboard type" that rise rather sharply then decline. The distance of the hills on any given rise or decline is perhaps a half mile at most; usually one quarter of a mile or less. It doesn't occur often. When it does though, I've begun simply shutting off the cruise control. On mostly flat stretches, everything is fine. Is this normal?