Odd problems--TP sensor??
I have a 2003 6.0 with about 150,000 and it has had this on again/off again problem, can go months running normal and then has a hissy fit. There are several symptoms--- driving at freeway speed it will suddenly not respond to throttle input for a bit then suddenly back to normal,maybe even dieing but normal after restart, or(again on freeway) without throttle input it will suddenly accelerate like you goosed it. My favorite is when at low speed it will suddenly start loping and sounding like a 2 stroke that loaded up, puking black smoke, I have to shut it off and after a restart it is normal again.
Batteries are new, FICM was replaced a year ago. Disconnected the tuner and same stuff. And never a check engine light.