rough idle resolved but have questions

[Peugeot Steve]

I have a 1985 Peugeot 505 STi with the 2.0L XN6 engine and automatic transmission (90,000 miles).

I've been using it as an almost daily driver (40miles each way to work). Love it. All of a sudden, it was stalling each time I came off the accelerator. The RPM would uncharacteristically drop to approx 500 RPM. I suspected bad gas? I researched this issue on this site. Boy, I suck at finding what I want. I was barely keeping it alive by feathering the gas;however, any quick let off the gas and the engine would roughly fall and surge and stall.

I changed the fuel filter (a lot of black liquid inside), but that didn't help. I thought it would have.

I used some water absorbing dry gas, but that didn't work.

I switched tactics and went towards ignition. I replaced spark plugs and wires (which I should have done when I bought it, but never got around to it). Neither of these helped.

I sprayed and wiped down the throttle body with cleaner, but that didn't help.

I disconnected the O2 sensor while idling, but no change.

After researching rough idling on the internet. I decided to try SEAFOAM. Look at this stuff on Youtube. My idle was constantly surging +/- 50 to 100RPM. The engine has always seemed rough. I poured 1/3 in the gas, 1/3 in the oil, and 1/3 in the air through the brake vac hose. I waited 5min, then restarted. Thank goodness I did this at night. The car smoked and puffed for 20min straight like a train--far, far worse than the video I watched. My neighbors would have thought my car was on fire. I smoked out the entire street. It was terrible.

Afterwards, the car idled as smooth as silk. It feels like a new car! I problably sound like an info-mercial. The before-and-after would knock your socks off. I drove around for about 45min. It was some of the most enjoyable driving I've had in the car. I'm a believer in this product. Smooth strong rev upwards...smooth lowering downwards. Like a new car. WOW, WOW, WOW!

I did find a bad solenoid after discussing tactics with Brian Holm today. The bank of vac/electric soleniods along the right fender. The first one (the one closest to the front of the car in the row of three of them)is the vacuum advance? With the top vac line removed, or the solenoid de-powered (disconnecting one of the wires), the car runs even better. If someone out there has a car like mine, please disconnect/re-connect this solenoid and let me know the results. Do I need to get another, or is this the way this is supposed to work?

Peugeot Steve