Dominic,

I had the same problem w/ my 86 Syncro. Eventually the problem got worse and
worse. As it turned out, it was a sticky starter solenoid. You can confirm
this by tapping on the starter assembly next time it happens. I used an old
golf club snaked through the engine compartment. It beats (no pun intended)
having to crawl under the vehicle to do it. Ultimately, the starter assembly
will need replacement. I don't think it is easy to find just the replacement
solenoids anyway.

BenT
San Francisco