'82 300CD rough idle at coasting 800 rpm only

[JellyDonuts]

Ok, I've scoured the web looking for something similar as to what's wrong and I need some advice. 1982 300CD, daily driver with 210k on the clock but a replaced engine by the previous owner at around 100k. (her husband ran it out of oil, is what I was told).

Problem: over the past 3 weeks the car has had a rough idle while slowing down/coasting below 30mph to almost a stop. Replaced a tie rod that was bad but it was still doing it. It's been getting worse, so I replaced the fuel filters the other day and threw in some diesel additive. The white bottle, they were out of Redline. The car has definitely picked up horsepower, but the shaking has gotten worse.

I've experienced rocking/vibrating engine in my 81 years ago that a valve adjustment cured, this is much worse. Only the harsh vibration at 800 rpms. Below at a stopped idle of 650 rpm, nothing. Anything over 800 or 850, it's smooth as silk. Very weird!

My conclusions are Damper Bolt adjustment or replacement, or it needs a valve adjustment. Although I'm sure it could use a valve adjustment I just don't feel like that would surface so quickly and now violently. I'm guessing the increase if fuel is exacerbating the issue, whatever it is.

Has anyone else experienced similar rocking? It started in what I thought was a suspension problem since the front of the car was swaying side to side from 30mph down to 5mph. Applying brakes or a little accelerator would immediately stop it.

Well, those are my 2 conclusions. Anyone have anything else or similar experience? Thanks in advance!