My first guess would be the throttle positiion sensor but Im guessing you are referring to that with the TBS is good. Maybe the pedal itself is a problem or maybe there is an issue with the Cruise control.
Has there been any new electrics fitted recently. I have heard of one person that fitted a mobile phone system to an explorer that had one of the power wires tapped into a cruise control wire. The thing stayed at part or wide open throttle on the highway and almost ended badly.
The dealer should have been able to sort this issue out really and don't go driving your vehicle until this is correctly sorted.
71 429SCJ Mustang