[Aerkuld]

I had exactly this problem on a Boxster. Check the micro switch on the clutch pedal. This is a normally open switch, but needs to be closed for the starter to work. This normally happens when you depress the clutch pedal to start the car, but the switch goes bad over time and the contract deteriorates. Disconnect the two spade connectors from the switch and temporarily connect them with a short piece of wire. If the problem goes away you have found the culprit and should replace the switch.

Hope that helps.