It only does this in the summer time I have also noticed it is when I have the a/c on too. I can let it cool down or when the temperture outside cools down to the 80's it starts back up and drives fine like nothing ever happened. This is the 4th summer this has happened. Never a problem in the winter time. Its happens in alot of stop and go traffic especially.
My BMW 328I shuts off after driving for about 30 minutes in 90 weather on 2000 BMW 328i