This is my first post to the group...I'm hoping cyber-space doesn't eat it
up!
Re the above heading, the same thing happened to my '86 Syncro without
warning.  I was not in my home town at the time & had my van trucked to that
cities' VW/Porsche dealer.  They ended up charging me $400 & on the way
home, my van hick-upped a couple of times & once back home, it died a couple
of days later.

The culprit was the fuse block...actually the multi plug behind the fuse
block.
The plug is yellow in color (block # C ) & I think the wire is red, and 10
guage. Pin numbers 16, 17 or 18, Im not sure ( I think # 17).
The block has pins & the multi-plugs have the female sockets...My mechanic
said he has seen this a couple of times on VW's.  I guess that over time,
there is corrosion or whatever that builds up between the contacts & it just
takes that certain vibration to shut things down!  The shop I took it to
only fiddled with the female socket & should have replaced the male part as
well.
To see if this might be the reason, you should be able to un-do the fuse
block with the 2 screws & wiggle the wires at the back of the block, whilst
trying the ignition.
This is an easy fix (cheap too)  Hope this helps!

Barry & Valerie Fulford
Nanaimo.B.C., Canada
86 Syncro
52 Chevy