Lemmee ask some questions again, answers to these could help diagnose... I'm beginning to think maybe there's a problem with the Vehicle Speed Sensor. Your SPEEDO is working properly, yes ?

It does this only when you are driving (vehicle in motion)??? and if you pull off the road when it starts this, and just let it idle ?

Does it stop doing this the instant you crack the throttle open again ? (just enough to get the TPS idle switch open?)

Are you ABSOTIVELY POSILUTELY SURE that she's not got an air bubble in the cooling system ?? and running hot ?? and that the T'STAT is installed correctly ? (i.e. air bleed hole UP, facing correct direction?)

I would still try the following test, easy to do. If it still acts up with these hoses plugged, then it's certainly not related to that temp switch (I'm still not sure it isn't, even though you changed it).

Pull the hose from the air tube to the switch and plug it. Plug the hole in the air tube. See if the problem goes away.

Does your truck have A/C ? if so, are the hoses to the fast idle solenoid routed properly ?