Hi folks, continuing the long slog to get a neglected old car back to usability. I had been making good progress but hit a bit of a snag recently and I'm not sure what to try next.

The car: 1982 300d, 120k on the odometer (no idea if it works). It sat for decades with I think WVO or biodiesel in the system and was pretty gummed up. It was running when I bought it but just barely, rough and it would not rev up past 1000rpm or so and could barely drag itself forward on a level surface. It did not smoke at all.

The symptoms now after lots of work (see below):
Engine starts well but will die immediately without throttle input, and won't idle without some throttle until its warm (around 80C by the dash). Even when warmed up if you rev it up and then release the throttle it will often die when it hits idle. It revs up fine once its warm, is a little laggy until then. Moderate blow-by (based on oil cap teakettles a little bit). A litlte smoke from the tailpipe, almost none once warm. Air filter assembly has a SERIOUS vibration when cold at idle -- I worry about my bracket. Engine sounds pretty good to my ear. Idles at around 700rpm per the tach, I have not checked it with a laser. The issues persist even if I idle the car for an hour so things like residual air in the fuel or bad glow plugs seem unlikely.

The major relevant engine stuff I've done so far and any notes:
- new fuel tank and screen (old ones completely gunked up and screen plugged, didn't even drip when i cut the line)
- tank vent line cleaned and checked
- new fuel cap gasket (they seem NLA now, i made one from 3mm viton sheet)
- new tank pads (they're NLA i made some from neoprene foam, see other thread)
- fuel sender rebuilt (seized, ground corroded away)
- steel intake and return lines cleaned out (they were clogged with black polymerized fuel)
- new rubber fuel lines under hood
- lift pump rebuilt (old one gummed up)
- new hand primer (old one leaking)
- fuel pressure regulator banjo rebuilt (old one disgusting, spring was too compressed to provide good pressure, couldn't manage to bleed the injector hard lines until this was done)
- new fuel filters (x2)
- diesel purge (x2 including letting system soak overnight with purge in the system)
- oil changed, oil filter changed (oil level was 3 inches above high on dipstick when I bought the car, not sure if overfilled or making oil -- oil was dark but did not look contaminated, not milky, and no visible metal fragments)
- air filter assembly rebuilt, new bracket, new mounts, new filter (old one had no rubber mounts left, was held down by baling wire)
- valves adjusted (all valves initially very very tight; looked nice under valve cover, camshaft lobes looked immaculate)
- timing checked = ~2 to 2.5 degrees (based on lining up marks on cam gear sproket spacer)
- new firewall throttle pivot ball, throttle linkages cleaned (old ball disintegrated)
- rack tension bolt backed off a little, it was almost fully tightened (i wanted to replace this but its NLA now)
- new battery (H8 AGM, old one stamped 2002)
- new fuel (with healthy dose of startron and marvel)

Engine stuff still to do or that I haven't touched at all yet:
- injector rebuild (I suspect they aren't great given the state of the rest of the fuel system; I have the equipment to rebuild)
- italian tuneup (brakes dont work yet so no road testing but I'm sure it would help, there has to be a lot of deposits to burn off)
- idle setting (idles at 700rpm roughly per dash tach)
- vacuum system (a lot of the rubber vaccum unions under the hood are cracked and suspect; the car does shut off normally with key and the locks do somewhat work)
- ALDA
- intake boost pressure banjo and air line (definitely clogged, I can see it plugged 8 inches up the line towards the firewall)
- plastic "clear" fuel lines (so dark I can't see through them but not obviously leaking. can't see to confirm no air leaks, i bought a couple sizes of tygon to replace the line with)
- crush washers on various fuel system banjos (the aluminum crush washer on main fuel filter bolt definitely leaks air bubbles when I prime the system, not sure if that means it sucks air also)
- throttle linkage adjustments (there isn't much play but I noticed the linkage to the inside of injector pump did not have a tightened lock nut on the upper end, suspicious, not sure what a long forgotten PO was up to...it does seem to have full range on pedal)
- injector pump drip timing
- compression test
- glow plugs (i bought replacements but hasn't been a priority)

As of now I'm mildly worried about air in the system, since I can't see through my "clear" lines and I know the fuel filter bolt does bubble a little air under pressure. I also suspect my injectors need replacement but not sure if that is causing my current idle/power issues. And I'm not sure if there is a vacuum issue that could be causing these signs, the vac system is complicated and highly suspect. I'd appreciate some advice about where to go from here. Thank you!