Hard to start or... not starting at all.
These were the symptoms of a 928s4 I know. All seemed fine, everything checked but no start. Using starter spray in the inlet did make it start and run perfectly until stopped again. After some time it was noticed that starting the engine and turning the crank around was kind a slow. Too slow. The engine had a hard time picking up on a combustion.
The cause? Bad contacts. Not sure which ones, the ground straps fro battery to chassis and from chassis to engine, or the contacts at the starter itself.
Anyway, all cleaned, sanded, tightened, and used some acid free Vaseline grease on it to prevent moisture form entering.