[DanEC]

It could be a weak starter solenoid or relay - probably affected by heat. If you can find a starter and alternator repair shop (kind of rare these days) they can probably trouble shoot it for you. You should check the voltage while actually cranking over the engine as this is a better test of a battery versus static voltage. I forget the exact standard but I think the voltage should stay up around 10-1/2 volts or higher if the battery is good. If it's dropping to 6 or 7 volts the battery has a bad cell or internal short.