[Mike Taylor 3601]

I would say it would just about have to be something with soleniod,maybe it's not strong enough or have bad connection at times,
Could be in solenoid where contact is,where main battery cable goes on soleniod back nut off slightly so you can wiggle the post in solenoid where main battery wire goes,then put battery cable back on and tighten,cable will be tight on hot post,between the 2 nuts,but you will be able to wiggle post on soleniod,what this does is let contact move around in solenoid so they make better connection.
Mike Taylor 3601