I have searched and read all I can see.

1988 300TE with 64,000 miles.
A/C not working.

Starts fine, runs fine until 85deg is showing, and then begins irratic misfire that shakes the whole car. Not constant, sometimes 3 secs betweeen misses and sometimes 10 secs.
The car is also alway very sluggish at low rpm.
Normal idle speed is showing 750/800rpm. When the miss happens, that idle drops by 100/200 rpm. Economy gauge normally reads halfway between the stop and the start of the red section.
It has stalled once in this condition but started again instantly.

Yesterday it did something different.
First thing was that the battery was dead. I think it was the alarm. I charged it for a while, disconnected the alarm under the carpet and it started instantly.

When I drove it, I noticed the miss was a little harsher, I was sitting waiting for my son, and I watched as over a few mins the idle speed crept up to 1000rpm...I noticed that the economy gauge was pinned to the left stop... ie max vac..... then "boom" the engine just quit.
No miss and stall, it was lights out, like I had turned off the key!
I restarted it and it fired instantly.
The idle resumed at 1000rpm and after a min or so it repeated the same thing.
It restarted instantly again.
Just after the 3 time it did this my son arrived and we drove away.
The miss and stumble was all the way up the rev range and continued all the way home.

I knew that the car had an issue like this, but am pretty handy, so was not put off.
I do know that I may have a vac leak as there appears to be a pipe missing from the valve cover to air filter housing pipe.
The OVR has not been replaced according to the bills I have, but the fuel injectors have, as has the potentiometer, ignition leads, cap and rotor.
The car sat for 12 years or so from 1990 to 2002, and failed its first smog badly.
Its most recent smog it only just passed.... HC was on the max at 105 and 81PPM. The PO said it was missing when it was tested.
I listened to the fuel pump/pumps and they sound a little odd to my ear. Kinda "gritty", not like I know Bosch pumps normally sound, but if there are two, maybe its me.

So, I know this is common, but with so much info, where should I start?
OVR?
Idle control?
Ignition system?
Vac system?
Fuel filters/pumps/relay?
Mixture?

I guess I need some help with the priorities to figure this out?

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2009 Mercedes ML320 Bluetec