I had that problem, too. Not always, mostly when I drove the car some km and
it was already warm.
My problem was a faulty brake booster. Due to a broken seal in it, the
engine could suck in air through the brake booster�s vacuum lines and died,
mostly when in neutral and rpm�s going down from normal cruise to idle.

I heard a small hiss coming from behind the instrument cluster. Changing the
booster helped solving the engine problem and also gave little more power to
the brakes...

Raimund
92 T3 LLE w Audi V6