I had a similar problem...turned out to be the dash pot on the throttle body. The rubber boot was torn, and I guess enough crap finally got in there to mess with it.

It took me awhile to figure it out, though, because it would only happen when the dash pot got hot. I inspected it a few times while the DP was cooler and it worked fine.

Then I pulled over to the side of the road once while it (the engine idling around 3000 RPM) was actually happening. At that point I was able to close the TB a little bit more by pushing on the linkage and the idle dropped to normal.

However, the dash pot wasn't done fighting yet <img src="/forums/images/graemlins/shiner.gif" alt="" /> It actually pushed the throttle back out a bit and raised the RPM back up to 3000 RPM. So, I took out the little stopper bolt (or whatever you call it) to disable the dash pot (so it wouldn't actually contact the throttle linkage anymore) and the rest of the ride home went fine. Once home, I sprayed a bunch of silicon lube inside the torn boot on the dash pot and re-installed the stopper screw. It has worked fine since. (I could drive fine with the dash pot disabled...if you do a search, you will fine a thread about the pro's and con's of disabling your DP.)

Also, there is a bypass inside the throttle body that allows air to go around the butterfly plate. The bypass is closed with the engine at normal operating temp. When the engine is cold, however, the bypass is open so that the engine will idle at high-idle instead of normal idle. Once the engine coolant heats up, it closes the bypass and your idle drops down to normal. (It doesn't seem likely that this is causing your problem, though, since your idle is so far above the normal high idle mark. But, this is one way air can get by without the TB being open.)

Stan