The solenoid on the starter appears to not be engaging correctly, so after withdrawing the key, the starter motor is still spinning without engaging. If you give it a (gentle) thump with a non genuine tool (steel bar) it stops and, with some luck everything works the next time starting is attempted. Can the starter motors be easily repaired or is a replacement part easier?
John 535is Sydney - 1989 (M30) Alpine white, black recaro, HIOP, Bilstien, EAT, 17" BBS.