Hi

1987 Syncro 2.1 sat for 2 years. Ran perfectly (smooth powerful etc)
until one day it didn't run at all.

Replaced my ECU (with a used one) because the van wouldn't start and
everything else checked out (to the best of my limited knowledge).

Fired up instantly with the replacement ECU. But now it idles at 2200.
At a stoplight it will stay at 2200 and smooth for a bit, then stumble
its way down to 1000 and sputtering, then clear up and run right, but at
2200.

Some lurching on acceleration, especially at low rpm. When it lurches,
the tach drops, and comes back up as the engine comes out of its stumble.

FWIW, the old ECU was the D series, the replacement is not. I did get
the VW "vanagon syndrome fix" pigtail along with my used ECU. Doesn't
make any difference whether it's inline or not.

Also FWIW, I have coolant leaks as well, so the system has been emptied
and refilled since it last ran well. I have heat, and it runs at the
right temp, so I assume that I'm bled well enough. But I offer this in
case it is related.

Any suggestions about where to start?

I bow to the wisdom of the collective.

Humbly
Raymond