Grant12,
I would agree that the TPS may be going bad but thought I would add this just in case....
A while back I removed the Throttle Body on my wife's Expy 4.6L. When I put everything back together I had a very high idle. It revved up to about 2200 RPM, then backed off after a minute and repeated. It kind of oscillated back and forth for a while before I shut it off. Upon inspection I found a vaccuum hose had been left disconnected on the passenger side of the TB. It was a large hose with a 90' elbow. I had missed it because when it came off it landed on a screw and looked like it was connected to the intake. Anyway, I reconnected that and it instantly fixed the problem. I will look at tit and tell you where the other end goes when I get home tonight.