[Bow Tie Scottie]

I had a similar problem a few years ago when my cruise control (not engage at the time) disrupted the throttle linkage on the carb causing it to race with high RPMs like you described. What was discovered is the lever that is connected to the cruise control snapped and lodge into the throttle cable. Scared me enough that I had to park the truck and have it towed. It was so strong, that I had the brake pedal pressed down and the truck kept moving!