what EXACTLY do I need...?!

OK...I just got paid...so now I need to buy whatever it is. The usual high idle problem. The sensor that always goes bad. '89 4.5. Here's exactly what it's doing. I need t oknow exactly what to replace. I know there's like 2 possibilities, but I don't want to get both if I only need the one. I got a shop manual, so I should be able to install it without much difficulty.

The car seems to start perfectly now since it's fall. It obviously likes cool weather. but once the engine warms up and you go to restart it...all hell breaks loose. Like today, it was high revving. Sometimes when you floor it/hit the brakes hard, it stops. Not today. Well...it did. It just stalled altogether. haha. My girlfriend was in the car and she was like "WTF?!" lol. It was funny. I started it back up, it high revved, it blew a little black smoke, and I went on my way.

When you're driving, the car bogs alot. It hesitates. Especially when you're pressing the gas. It sputters sorta. Very hard to maintain speed. It does this at all speeds. Everything from a slow creep, to highway.

The car ,when you actually rev it, sounds a little funny. Instead of a normal *whoom* it's like *wh-o-ooom*. I don't know if that's just all of the 4.5's, or if it's just becasue of this problem.

When it does the initial high rev and idle after a warm start up, the check engine light comes on. Sometimes it'll go off after manually revving it a few times, but then it'll just come back on again after heading down the road.

That basically sums it up. I already tried t oclean the throttle body and stuff like that. The only thing I didn't do was a tune-up. It doesn't seem liek it was done that long ago, but this DEFINATELY seems like a sensor problem instead.

Please, ANY info would be amazingly appreciated. I need to fix this ASAP!! I don't want my Cadillac to crap out on me...