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IAC doesn't really matter here, your problem is present always, not only while idling, if I read your post correctly. You may want to clean it anyway, though. Clean the throttle body as well. And check the PVC system (flame trap, nipple, small diameter vacuum hose). That covers the basics. May not change a bit in your case, but why not do it just to sleep better.
Check if your spark is good - inspect plugs and distributor cap & rotor, replace if needed. You may need to change the plug wires and coil wire if they are old. I'd also try to rule out the exhaust side first -- plugged cat converter and/or muffler may cause a lot of grief. If you know how to disconnect (and secure!) the cat converter, go for it just to rule out the cat & mufflers. Even better if you can measure exhaust pressure before the cat converter.
As soon as the spark and exhaust are taken care of and symptoms persist, you may have inake-side problems, assuming your electronics are happy (i.e. no codes and no check engine light).
That means either fuel or air, and as far as air is concerned it could be too much or too little of it.
Fuel-wise: on my list would be fuel pressure regulator, dirty/bad injectors, injector grounding (very important).
Air-wise (too little air): dirty air filter, clogged air intake (before the filter!). For a quick check of air intake & cleaner, try disconnecting the intake hose from the air cleaner box and try running around the block. A very bad thing (tm) to do, but if you're careful (as in not going on a dirt road or behind something that raises tons of dust into the air) you won't ruin anything. If things get better, you may simply need replacing the air filter and making sure that air intake is not clogged.
Air-wise (too much air): vacuum leaks, throttle body and intake manifold gasket leaks. Make sure that all the vacuum hoses are O.K. There are several ones that you can safely remove and plug their throttle body/manifold ports w/o impacting engine performance. If things get better after removing vacuum hose and plugging the port, assume that either that hose is leaky, or something on the other end of it leaks. Start with big vacuum hoses (say brake servo) first. To check intake side seals/gaskets: spray water on/around tb and intake mainfold gaskets, and onto bases of injectors (injector seals!). If idle changes - that gasket/seal is faulty. An extreme case would be a crack in the manifold.
Cheers, Kuba
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