1991 Porsche 944 starting

Hi I have a 1991 944s2 soft top, it starts from cold fine, drives fine, if I stop the engine
Then leave it for an hr or more it starts fine, if I leave it for 5 to 10 mins it starts ok, if I leave it for 20 to 30 mins it just will not fire does not even try, all turns over fine ( it has always turned over quite slow it probably sound as if the battery is a bit flat but its not may be the starter motor needs overhauling? ) Then after trying to start it for around 10 mins it then starts runs a bit rough for 10 seconds or so then can drive off runs fine, I had the car serviced in Nov 2009, changed the DME relay, new dis cap, new arm, new plugs and leads, cleaned the TDC sensor at top of fly wheel, still does it, on reading forums I think it could possibly be an fuel injector leaking into the bore after stopping the engine can this make the spark plug wet and there for no start? But I would have through it would have started on 3 cylinders until the engine cleared the un burnt fuel then the flooded piston would fire? But I am only guessing can u help.
Regards Andy from London SE9