Hi,
My 85 Westie would not start after a three-four hour drive on the
interstate...this had happened before around town but was cured with a
new battery. After about fifteen minutes it started...then it happened
again about an hour later and would not start at all until I jumped it
forty five minutes later....the additional voltage must have helped
but it wasn't the battery...there was nothing when I tried to crank it
except the instrument lights did go on...My local shop ran it all day
with no luck repeating the symptomes...even drove it for a couple of
hours...must have been the highway miles/speed...They checked the
grounding harness and the wiring...and the starter/solenoid
{apparently the solenoid would cost more to replace than the entire
starter/solenoid combination}...no luck...the starter appears to
register the same on the gauges whether it's hooked up from the van or
direct...So...the only option is to replace the 160$ starter/solenoid
and see if that takes care of it on my next long trip...the theory is
that the wiring in the starter must be getting heated up during long
use at 70-75 mph. Any other suggestions? Thanks for any advice...I
must do something...its no fun being stranded... 
Rick