Greetings all,
I apologize if this has been asked. I've been poking through the forums for about a week now, and haven't found anything that quite seems to fit.
I have an 83 380SEC that I got because it wouldn't run right. The previous owner had had it into the shop numerous times, and it would run good on some days, and then randomly, it would surge down the road as if someone had popped the clutch too fast (it is an automatic, no clutch. Just that's what it feels like). The engine would not hold a steady RPM.
I got it, went through with some starting fluid and found about a bazillion vacuum leaks and replaced the hoses. It behaved better, but still not right. replaced the idle control valve and relay, and it got a bit better, but still not right. replaced the distributor, and then had a different shop finally tweak it the rest of the way and it ran great for about a year.
Then it began to run REALLY rich. Of course, it did this about two months before it needed a smog check. The machine still ran steady and got me back and forth to work, it just had gone from 19MPG to 16MPG.
I took it in to get it smogged and figured they'd adjust the idle. There is only one import shop in the area. They replaced a bad hose and tweaked on it and got the mixture down to a reasonable level and it passes smog.
Unfortunately, now it won't hold a steady idle, and going down the road it surges. Basically, a similar problem to what it had when I got it.
Symptoms:
When I initially cold start it, it holds steady at about 1000RPM, and the "economy" vacuum gauge is about 3/4 to the good side with the needle steady. If I let it sit until it starts to register on the temp gauge, it drops to about 750RPM, and begins to run more rough. The vacuum gauge strays towards the center, but still holds a steady needle.
As it warms up further, the RPM drops to about 550RPM, and she runs really rough. The vacuum gauge slowly moves to the bad side and if I don't do anything, she'll die.
She generally fires right back up, holds steady at about 750RPM for a few moments and then creeps back down and dies again.
If I keep her running till she's about 60C on the temp gauge, then she'll idle rough at about 550 RPM and won't die.
If I crank it up to about 2000 RPM and hold the pedal steady, she holds for a few moments, and then decided that she wants to go to 1750RPM, or maybe 2700RPM. There doesn't seem to be any reason or rhyme to this. If I reset my pedal depth to 2000RPM again, then it'll hold there.
If I try to drive down the road, she just surges.
I don't know if it means anything or not, but when idling at 550RPM, and running rough, the vacuum gauge is just to the good side of center. If I raise the RPM to 2000, it goes almost all the way to the good side.

This time I can find no vacuum leaks.
distributor, idle control valve, idle control relay were replaced previously as noted above.
fluid levels are all good.
It ought to be some kind of adjustment, since it went in running steady, but rich, and now after being adjusted to pass smog, it's running poorly.
I don't have a manual (and can't afford one at the moment, even if I could find one), so I have no idea where the mixture adjustment is, which was my first thought.

If anyone has any idea what might have been tweaked and how to tweak it back, preferably without resuming the intense sucking of fuel, my suburban which has been pressed into daily driver duty would really appreciate it.

Thank you
John