The Idle Air Control is the only component that could do what you are describing, keep the engine at "cruise". Troubling to me is that it seems to be more than simply a sticking valve. There is no time where the IAC should be wide open in order for it to stick open. It seems like something in the control system is telling the IAC to be wide open.
Disconnecting the IAC will be an important start to diagnosis. What comes after that is pretty murky to me.
R.
Some ECUs have a throttle follower parameter that activates with rapid throttle open/close, to give a ramp down to idle. As suggested, a failing tps could trigger this parameter.
I have no clue of what ecu we are talking about here.
