I agree that you've ruled out some possible sources and have narrowed it
down to the next likely candidate (in my opinion at least) which is wiring.
I replaced the cable that runs from the battery to the starter on my vanagon
this summer (after replacing batter, starter, cleaning grounds, cleaning
switch, etc.) and saw a big improvement.  A few times since then though I
have seen a slow turn over though it is rare.  I'm guessing that my
remaining problem is the wire that controls the engagement of the solenoid
switch.  I guess it could also be the switch itself too.

In any case they sell a relay kit on the bus depot site and possibly
elsewhere that I just bought and am going to install this week.  It will
take a few months before I start to feel confident that this has fully
resolved the last of these symptoms on my van, hopefully it will.  If not
I'll replace that last wire and the switch and start looking for what to
replace next.  I guess I'll add a switchable lead between my house and
starting batteries or just start parking on hills like I did in my old 6
volt VW's..

If you decide to replace the main battery cable that runs to the starter and
need some help on how to do it I can share my own experiences.  I didn't
have to drop the tank (used a pull cord) and the job actually went pretty
quickly.  Others may have even better procedures but be sure they are fairly
quick and easy because they can be.

Good luck, and let us know if you can contribute to the list of solutions to
this fairly common problem.

Jonathan Poole
'83 AC Westy

.....

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.