Hope your problem is fixed already.
If not start back on the basicis again, I know you have heard this allready but any contamination will slow the flow of power to the starter and that solenoid.
Battery cables good connection tight fit, check the ground to the frame or block, clean them with fine emery cloth and all the surfaces they touch. It may evevn be a weak battery because of your weather.

Hi !

No i have bougth a new solenoid but haveŽnt mounted it yet. (Beacuse the weather realy sucks now.) So the problem isŽnt solved yet.

Yes i have gone there a couple of times now, and i have measured the cells of the battery, and it shows nothing wrong.
All points that connect the enginge from the battery, including ground are polished as mirrors

The only thing i haveŽnt checked are a tip from "Padgett" regarding a jamming clutch inside the starter, thereŽ* a good chanse that this might be the problem.

So as soon as the weather shifts to something that includes the sun, i will dismount the starter and check it out.

Regards
Peter