I had this problem with my 85 Westy.
First I replaced the O2 sensor on the exhaust.  No change.
The I took it to a very experienced german VW mechanic.
He found the mass air flow meter (aluminum thing to the left of the
air filter box) was way out of spec (ohm meter) and tried a replacment.
That fixed it.

370.00 dealer
175.00 - 250.00  bus depot, volksmotorsports etc.

Best of Luck!
- Wil

For the last couple of years now I have had a problem with my 93
Vanagon  stalling.
The problem seems to happen at random. After the stall the engine will
not start, and it seems like it is flooded (smell of gas). After about
half an hour I crank it over, A puff of black smoke, and away we go
running like a champ. The problem seems to be becoming more frequent I
have replaced the Temp coolant sensor. I also picked up an ECU from a
junk yard and threw that in for a try. The Oxygen sensor was replaced
about 1.5 years ago.
Any Ideas ??