'86 Westy

For those that don't remember:

I had serious problems with the van, tach going to zero then right
back up again.  I did everything and followed all the list members
advice except for the ignition switch and distributor.  Those last
two items I did shake rattle and roll while the van was running, to
no effect.

Finally I took the van into a VW shop (it had to be towed).  This guy
put in a new OEM O2 sensor ($$$$$ yikes).  A new ground strap and
played with the ECU.  The van is now running fine.  I personally
tested the old O2 sensor with a meter, it seemed fine to me.  I also
cleaned all the grounds and used an OHM meter around the engine to
ensure resistance was low...it was, near zero on the ground strap.

When I got the van back, the ECU was not screwed in, it was lying on
its side on the floor....and the van is running perfectly.  So, given
that my tests of the O2 sensor and grounds were valid...how would the
ECU lying on its side fix the issue?

Thanks

Gregor
   P.S.   I am now frightened to put the ECU back!