I'm afraid it could be a dozen things. There are a few threads on here with people having similar problems, some resolved, some still outstanding (like my own).
- MAF sensor (try unplugging the sensor part and see if runs more smoothly without)
- lambda sensors or connections (do not always give a Lambda error. Welcome to crazy Alfa Romeo costs to replace)
- spark plugs (but I see you've already replaced these)
- coil packs (swap them round, see if misfire moves cylinder)
- ECU or ECU connections
- wiring loom
- air/vacuum leak (check hoses, joins and look for cracks in manifolds)
- damaged exhaust (visual inspection may not be enough)
- damaged cat (knock it gently with a hammer, does it rattle?)
- stretched timing belt (sorry, it's still possible even without apparent timing errors)
Any other symptoms to report? Poor performance? Failed MOT emissions test?
Have you had the diagnostics checked with an Alfa 'Examiner'?