jitty2

So I have an 06 sierra 1500. The trucks shifts fine but when cruising in overdrive it will drop down to 3rd gear then when i give it more throttle it goes back into overdrive. I took it to a tranny shop and they took it for a ride with computer hooked to it. They said it was shifting when the computer told it to. He thinks its the throttle body and tps. I have no check engine light on. Any help would be great.