Originally Posted by ransoman
'tis a simple Maf problem by the sounds of it.
On the induction pipe you will notice a wire plugged into a sensor. Unplug that, if all is well afterwards then your MAF needs replaceing (Tip, only buy a geniune bosch maf).
This fault does not show up on a typical diagnostic scan.
this is the best advice to start with - nothing else should be done till you try this - its a FREE test too.
btw, your mechanic clearly has no knowledge at all about Alfa's - you need to find one that does for next time
let us know how the MAF test goes
(while MAF unplugged your engine management light will come on and idle will be wonky - but you can test the rev range to see if it improves - all will be back as is once re-connected)