Mark,
I have a similar problem with my 88 4/4. It is not consistent enough for me to worry about though. In Western Australia, cars less than 25 years old have to have an immobiliser fitted, and I wonder if that is the cause for me. If I leave it for a couple of minutes it seems to start OK. I do not think it is a battery issue, as it does not struggle to crank. It is more like an isolation issue of some kind.
In my experience, starter motors usually "Click" if they are having an issue with the bendix engaging, and that is caused by lack of battery power. Perhaps you should replace your earth strap, it may have a fault, there was a thread recently I think on this
Have you had your alternator checked?
