[dw1]

I've had similar problems on several different cars (no surprise - they were British sports cars) and it was traceable to one of 2 problems:

Bad solenoid or bad ignition switch

The only exception to that was an older American muscle car where the the insulation on the wire from the battery to the starter was worn and the wire was shorting to ground on the inner fender. That was a scary one, because after the car started, the wire insulation started to smoke and the engine kept running (but very badly) after the key was removed from the ignition. As quickly as I could, I removed the wire from the battery terminal, limiting the damage to just the wire... and the solenoid-starter motor assembly.

Luckily, I have never had any problems like this with my 911.

Fyi, I did do some work on my 911's steering column & ignition lock last year and it was not that bad. Getting the tamper-proof screws out was the most time-consuming task.