Bought this STS about a month ago (ok, maybe 5 weeks), and shes had a rough idle since I got her. I had a 100k mile service done at the dealership before I took her home (no charge, thank-you-very-much). I specifically told them to use Bosch Platinum plugs, and to make sure the gap was correct (because the first time you see the gap on the specs, you think it cant possibly be right...right?! HA HA!!). I thought new plugs would take care of the rough idle. Nope. So I took her home and took off the EGR valve & cleaned it (just a carbon dust build up, but it didnt seem to be sticking open). I'm a tad bit knowledgable about things under the hood, but not experienced by far! Was hoping y'all could throw some suggestions out there for me to try. Obviously, I'd like to start with the 'little' things that may fix the problem first. I'm an optimist at heart

 

Oh yah, no codes. Also, she does it whether she's cold or not, but it does seem less evident when shes warmed up.

 

I would clean the plunger on the IAC valve, also the intake plenum gasket rips on the bottom check that

 

I suggest inspecting the coil packs. They are bolted onto the rear valve cover. This placement is idea for collecting moisture and water. Pull the caps and check the posts for rust and corrosion. You are best to repace the pack if you see rusty posts but a good cleaning might also work. There are 4 packs, last time I had to buy one for this vintage Northstar it was $88. Replacement is just screws and bolts.

You might also check your ignition wires, at this age it would not be unexpected to have some leakage. Start the car at night and use a spray bottle with water to spray around the wires and coils, if you see blue arcing you know you have faulty components. Last set of wires I bought from dealer was close to $200.

You could also check for vac*** leaks, it wouldn't be unexpected to have some cracked and dry hoses at this age of an engine.