This usually happens when the speed signal is not clean as cruse control relies on the speedo signal, the speedo signal comes from the wheel sensor to the abs and then the ecu then to you your dash. but it can also go the sunroof unit window controls and the cruse control as there is a lot of links these can get confusing in the diagrams trying to follow the wires. best just to try a clean. remove the connections for most of the parts i have listed if you have a jump in the speedo as you look at it or a a wave at steady speed this could mean the wheel sensor or the abs unit, check the connections and clean them.

In my opinion, this is nothing to trifle with. It's a safety issue. I'd take it to the dealer, pronto.