Hm, i had a problem familiar to this too. But it happened only for the three times. Mine is 2.0 16V too.
A month after i bught a car more than half year ago, i started the engine, but it idleed wierd - low and ?restlessness?. Then i turn it off, and start again, and it was OK. I think, someday in february, i started it in the morning in garage, and even second starting up didn't help. I drove to work, and after 500 meters the sound was back normal like usual (as it could be with broken manifold). Third time it was last week. It was realy hot - 30 oC, Aircondition was showing 38 oC, the car was parken and the sun was hitting left side of the car. I started engine, and there came that familiar wierd idle (i usualy wait few seconds of idle, and then drive away). I turned off, and was expecting new fireing up will help. But it didnt, again strange idle, and engine electonic red light (injection) was on, and didnt fade away when motor was runing. I wanted to drove away, but when i pressed a gas pedal, engine died (stamp out, extinguish). I turned it on (injection light turned off after few seconds, like usualy) for few times and easily stopped on gas pedal, but nothing, it stopped. Then i turned it on and let it idle for half a minute, now it was working again normaly. So i drove away.
Next week i'll probably go to my Alfa serviser, and i hope the red light recorded some usefull fault.
Hm, the fact that engine died when i pressed gas pedal, i think this means too much fuel comes to engine?
There is one fact about my car, that could be a reson: the idle electric motor (motor minimum) is on his way to die. The car's idle rpms are ok and steady, but the problem is at accelerating from low rpms (under 2.000rpm) - engine tug (not accelerating lineary). I learned to drive that you cant noticed it, but i have to change it anyway, when i am driving easy and want to pull from low rpms, that chocking come up. The main problem is, it's damn expensive (130 pounds, here 150EUR), i would rather make chiptuning for this.
And broken manifold now have a friend, a broken rear exhaust tube. I realy have to manage this, but, it makes realy great sound when gas pedal is on the floor (K&N 57i too)!!!