Cruise Control problems of the 2004 Mazda RX8

The contact owns a 2004 Mazda Rx8. The contact states that while driving with cruise control set and hitting the brakes to slow down or stop , and if pressing the resume button ,the car will actually set itself 5 mph more than what originally had the speed set for. Once the car goes to 5 mph over the speed the car was set for the car will act as if it was trying to make itself go back down to the speed that was originally set ,and the vehicle will jerk violently. The contact was taking the vehicle to the dealership on Thursday to see if they could diagnose the problem because this has been occurring for a couple of months. The only way to make this problem stop is to turn off the cruise control and reset it and then it will stop. This problem with the vehicle at highway speeds will happen at about 60% of the time that the cruise control is set. Also, the engine fan will stay on all the time. The dealership is unable to duplicate the problem. Therefore , it cannot be fixed at this time. The electric fan will actually stay on while the vehicle is turned off. Then by turning the ignition back on and off it the vehicle will stop running. This mostly occurs if the vehicle is driven for about 30 minutes.