My 2.8Q started to do this a while back. Turned out the water pump was seizing up. Luckily it developed a leak so had to be swapped out before it completely seized and stripped the cambelt and most of my 30 valves. Audi had changed the cambelt at 75k miles but had failed to change the water pump at the same time saving me a valuable £35 on the £800 they charged for the cambelt change, w@nkers. Had to have the whole lot done again 10k miles later as changing the water pump involves the same amount of work as the cambelt. Luckily I'd found a VAG specialist by then who did the whole lot (water pump, cambelt & tensioner) for half the dealer price. They showed me the old pump and you could hardly turn the thing by hand so no wonder it was causing the engine to stall at idle.
I'm not saying this is the problem with your 2.8 as it could easily be something else, but worth considering as the consequences don't bear thinking about. If the cambelt has been done you need to check the water pump was done at the same time. If it's due for a cambelt change then get it done ASAP along with the pump.