I have a 5.3L GXP that just started and intermittent problem. At a stop at idle my speedometer will jump up from 0 to 40-50 at random. At the same time this happens the radio volume increases and decreases accordingly. If I begin to accelerate at the time this is happening it appears that the transmission is not ready to move and sort of slips into gear. Anybody ever experience this wierd thing. It usually occurs after the car is sitting for several hours and eventually stops after several miles of travel.