Jumpy Speedo Problem

I have a 2000 Cabrio and after driving 65mph or higher for 20 minutes or more on the interstate, the speedo needle wags back and forth. (This never occurs driving around town at lesser speeds). All else is fine, the car runs fine, the tach is normal, the odometer correctly records mileage. If the cruise had been on, the car will lurch and the cruise will come off. Once this occurs, the cruise cannot be put back on. I have had the speed sensor replaced, the speedo drive replaced, and the dealer tells me the computer says nothing is wrong. Any thoughts? THANKS!!