This can be a problem caused by wear in the starter motor oil seal which allows oil to leak past onto the brushes and the symptoms are exactly like those described - push the start button and the engine turns only slowly, if at all, as I found out on my 916 a couple of years ago.
Despite this not being a service part the seal can be removed and a new fitted, a new seal can be had from any specialist seal supplier. its a bit of a pain to do as the left casing has to come off to get the starter motor out but at least you can check the crank end nut is still tight at the same time!
Not a fix that some dealers like to advertise as they like to sell a new starter motor and throw away they old one. Waz that last bit a bit cynical - I dont think so
