This happened to me once and I put it on an exhaust analyzer trying to
figure out what was going on.  The O2 readings and CO readings were both very
high!! Highly irregular having a rich indicator and a lean indicator. This
brought me to the fact that the ECM was firing injectors erratically.  After
removing the ECM harness @ the ECM, I started measuring all the resistance
values according to the Bentley manual. Bingo pin 19!!!   This wire runs
along the O2 sensor signal wire and acts as a "shield" against RFI according
to the diagram supplied by Bentley. It seems to me it also acts as a ground
for the ECM cause when it doesn't have a good ground with the block (.3ohms
or less) then the engine will not operate under any kind of load unless you
turn the ignition off and restart which will only buy you time (2-3 minutes.)
The reason this happens is when the ECM goes into "closed loop" it relies on
the O2 sensor and that particular ground for dependable signal. My guess is
that under open loop, those signals are ignored, hence ok drivability. The
fix for me was to relocate that ground from a bakelite housing to a nice
metal distributor a couple inches away. I haven't had any troubles with mine
since that occurence.  Just an idea, I think this ground location is very
succeptible to intermittent opens in the circuit due to its location and may
just fix your van and make you realize that these things aren't that comlpex
after all. Eurovans on the other hand. Anyone ever replaced a clutch or
performed head-work on one of those things?? Wow
Anyways good luck.
-Jon Hess
91 Syncro
66 Beetle