Could well be the idle control valve, but Alfa's diagnostic check should have picked this up. Also, if the lambada sensor is a bit off it could well be that, though again the diagnostic should have picked this up.
Another, cheaper to solve solution would be a leak in the air supply pipe between the air mass meter and the engine intake as if this has occured the amount of air entering the engine wouldn't match that which the AMM is telling the ECU the engine is receiving.
I assume that you've checked the basics, such as spark plugs, leads, fuel supply, air filter etc..
I hope this gives you a few ideas!
Warm Regards,
Dan