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Thanks Civvy, I had just replaced the IAC and engine air charge temp sensor when I read your post and the problem is fixed. The IAC had been cleaned, but as I read in a few different places, sometimes they just need to be replaced. That was really the last item on my list as well. I am still not understanding how it affected the whole RPM range. The car now holds a solid idle and performs better during cold start as well.
Are you getting a check engine light? Codes? Are all the connectors plugged back in from your previous maintenance? Sounds like something to do with the crankshaft sensor (does it have proper gap?) since you do have spark. Possible spark is not being applied at the right time.
After a start attempt, pull a few plugs and see what they look like. If they have fuel on them, you can start looking at the ignition system. If no fuel, check the actual pressure at the regulator. If you disconnect the vacuum hose from the regulator, you would be getting full pressure for testing. You would then need to check the specs for your car to see what the idle PSI and WOT PSI (no vacuum) should be.
It is likely there was no anti-seize on the threads. Hopefully you put some on the new plugs before you installed them. Required for steel plugs in an aluminum head. The rust there is just cosmetic.
I would troubleshoot the ignition switch with a meter and make sure you have power going where it is supposed to in start and run positions. Because what you are indicating is essentially the ECU and fuel pump are not getting power. Short of the main relay going out, the ignition switch would be next. Hopefully you have an electrical manual for the car.
If you have replaced the main relay, I would look at your ignition switch. Jiggle the key around and see if it will start or power your fuel pump.
Since you appear to be getting codes for most all the cylinders, I dont think it will be ignition related. I would look at the fuel system. Check the fuel pressure regulator for idle PSI and WOT PSI. If you havent done it, run some seafoam or equivalent fuel system cleaner through it.
If you see oil build up on the edge of the valve cover and below it, I would say so. On a vehicle of this age (especially if the gaskets have never been replaced), it is almost a certainty. You can try to snug the valve cover bolts (don’t over tighten) and degrease the engine and see where that gets you.
You can use WD-40 (or something a bit better like PB-Blaster). Let it soak for 15-30 min. Make sure you are using a ‘flare nut’ wrench. Standard wrench will round the nut. From the drawing above, it looks like you should be able to put a wrench on the valve and then loosen the brake line nut. Then you should be able to remove the valve.
Likely valve cover(s) leaking engine oil on the hot exhaust. A mechanic could ID that pretty quickly.
What codes are you getting? You are saying that you don’t feel the engine running poorly but you are getting misfire codes? Is that correct?
Check to make sure that the vacuum line to the MAP is good with no cracks, etc. If it looks questionable at all, replace it. I have seen these cause issues under acceleration where a vacuum line crack is opened up with engine movement. Then, you would need to check the MAP output voltage using a vacuum pump to rule the MAP good or bad. Check to make sure you have 5V source as well. Have you had any work done to the car recently?