2004 Toyota Tacoma throttle races while stopped.

2004 toyota tacoma pre-runner, 40,000 miles, V6, with cruise control and 4 doors, and NO floor mats.
I just bought this truck 3 days earlier from a toyota dealer, so I have no past history.
While stopped at a red light, with my foot on the brake, engine revved up, and rear wheels started spinning. I then with foot still on brake, put truck into neutral, and rear wheels stopped spinning, but engine started revving really fast. After a few seconds, I turned the ignition key off, then started the engine again and everything was back to normal.
I took it back to the dealer, they say they can't find anything wrong.

What is the likely cause of this? Cruise control malfunction, or something else?

Any help would be appreciated, so I can have more info when I talk to the dealer again.