Thank you for the tip on how to measure voltage drop.
I like this a lot since I can do this at home in the garage without having to wait for the problem to show up again.
And I can use the same wire for measuring as for getting full voltage to the solenoid if needed (making sure to work safe of course).
If the voltage drop shows acceptable numbers I can move on to restoring or replacing the solenoid and/or starter motor.
I have to say that it is fantastic to be able to get great free advice like this from experienced people.
Makes me think that there might be hope for humanity after all
/Johan