I recently bought a 1990 Cadillac Sedan Deville with 80,000 miles. When I bought the car, it had a rough idle/miss. The guy I bought the car from had it tuned up with new plugs, wires, distributor cap, new filters, etc. Still it had a miss. The check engine telltale was on also. When I ran the codes, I got an E48.
So, I removed the EGR valved and cleaned it thoroughly. It seemed in pretty good shape, but the tubes under the throttle plate were completely clogged with carbon. I cleaned those and blew all the passages out with compressed air. Also, I cleaned the bottom of the throttle plates (they were pretty gummed up also). I cleared the codes on the ECM and nothing else has turned up. However, it still has a rough idle--especially when cold. After I've come off of a freeway run at 70mph or so, it runs great until the next morning. I've run a bottle of injector cleaner through the tank also, but that didn't seem to help matters.
Fatten the mixture very slowly with some sort of additional fuel like propane and see if the idle smooths out. If so, the engine is running a little lean and you're getting a lean miss in at least one cylinder, or there's an ignition secondary system fault that gets covered up with a fatter mix. Either way, I would suggest to get the car hooked up to a scope with a tech that knows how to read one. He will quickly be able to diagnose the miss, or at least narrow it down to a A/F problem or ignition problem.
I have owned my 90 Coupe since 92 and it is the most reliable vehicle I have ever owned, I just hit 198k a couple of days ago, this car has always been top notched maintained and you will not find another one with the same mileage that is that clean, anyway, it also has had a rough idle since the day it rolled out of Cadillac, I don't know how rough your idle is but mine sometimes seems to have a miss, it comes and goes as soon as I hit the gas it smooths right out, I had to change (not clean) my EGR valve once because it was setting a trouble code and causing a rough idle, the fuel pump can also cause it, but the question I have for you is, how is your altenator, and how are the grounds? That stuff can make a huge difference with computer controlled vehicles. If your altenator's output is off by even 1 volt, it could trip up the computer in turn effecting your idle, anyway, I just figured I would offer my thoughts, lemme know how you make out.
1990 4.5 Deville rough idle/miss 
