I would definitely try the Hard Start Relay that a few others have advised.
I had a similar situation with my '87 Wolfsburg that lead me to make the
prototoype of the Hard Start Relay that I currently sell.  The main
differences between the relay that I sell vs. the standard WR-1 that has
been around for 30+ years is the fact that I use weather resistant (I
hesistate to call anything electronic waterproof) relays and OEM style
connectors and insulator housings.  Full details can be found here:
https://sites.google.com/site/vanagonheadlightrelays/hard-start-relay

Jay Brown
'85 Zetec Westfalia
'87 Zetec Wolfsburg

Date: Mon, 14 Feb 2011 13:19:30 -0800

From: KK AL <kkalnsf@GMAIL.COM>

Subject: Hot start problem

After our 87 Syncro gets hot and we turn it off, it won't crank. The dash

lights up, the fuel pump spins, but not even a click from the starter. We

replaced the battery, starter and ignition switch. It solved the problem for

a while but it's happening again.

Any ideas on what to do next?

Thanks in advance for any suggestions.