I have a 2.0L 2000 Beetle which in the morning starts fine(Battery voltage is 12.4V), when I leave it idleing in the morning after about 5 mins it stalls and it won't start again battery voltage is only 11.7V (I put a new battery on last week). Lock car and come back in an hour and it starts again. Last week a helpful family member crossed the jump leads and tried to start it, Diagnostic check is all clear now. If the alternator is charging the battery would it make the car stall when running?

Any help on what to do would be really appreciated

sounds almost like high resistance heating up a component or something and after cooling down it's ok to start. if alternator was bad, it would just drain battery, but not in 5 minutes. check out put of alternator voltage shold be about 13.3volts or little higher depending on your vehicle. if it is, I think something else is causing stall. does it over heat. coolant level allright ??? check all basics first just to be sure.

Had to jump start car this morning voltage only 12v exactly . I had a meter on the battery with everything switched off, as soon as I removed the jump leads the battery voltage started losing quickly, down to 9.5V in about 30secs. When I revved the engine the voltage quickly went up to 14.26V (a little high?) and was stable then around 14.2V.

I loaded the heaters up one by one, the minimum battery voltage was 12.8V with everything on.

The only thing I can think it is, is a dodgy alternator that needs revving first thing to boot it into action. Does this sound feasable, I cant check the alternator that quickly