I'm reading this with interest since my v6 has the exact same problem. Aux belt went, engine overheated, new belt fitted, start up and it runs like a box of spanners at idle speed. 1500rpm and she sings like Catherine Jenkins.
I have been advised by Alfa man/mate that it could be: failed head gasket at worse, next, failed coil packs, next, failed/fried spark plugs.
Had it on emmissions test today and indications are NO gasket failure or misfire and if there is a misfire its a weak rather than failed plug. Fuel consumtion is 100metres/gallon at the mo.
If I get a soluiton before this thread ends,I'll post up.