Okay, I pulled the pulley belt off the compressor - pulley wheel spins and spins freely.
So the stalling is only occurring whilst the compressor clutch is engaged - A/c is on.
I recall on some older vehicles there used to be a solenoid that would trigger upon A/c clutch engagement and up the idle a wee bit to cope with the extra engine load. Is there something like that on a Porsche engine fitted with A/c? That maybe is disconnected?
Thanks.