Replied to a very similar question a while back and it seems in his case I was right and he was very happy to sort it.
Now of course this might not be the case with yours, but worth a try.
I had this on our Scirocco, and as you say it drives you mad !
Mine and the other guy's on here was this.
A blown diode in the alternator.
Most of the time the engine stops and the alternator stops with 1 of the OK diodes making contact = no problem.
But sometimes it stops with the duff diode making contact and this duff diode can quickly drain the battery.
I identified it by connecting a current meter into the feed from the battery and slowly turning the engine over with a spanner WITH THE IGNITION OFF = SAFETY !!
At one point in the rotation the current meter read a big drain.
Swapped alternator for a spare I had = all OK
If yours does this I am pretty sure you can change a diode quite easily.
Note, the diodes are not the same thing as the easily detachable voltage regulator / brush pack on the back of the alternator.
Hope this helps
John