Cruise Control problems of the 2003 Volkswagen Beetle

When cruise control is engaged, suddenly, without warning, the vehicle will accelerate at ever increasing speeds until disengaged. This happens intermittently, without warning and seemingly without a common denominator (such as 30 min after engaged or at a particular speed). It does happen with more regularity exceeding speeds of 70 mph on the highway, but it has also happened at 45-50 mph as well. The dealer was unable to duplicate the problem so subsequently did not repair the problem.