The most common problem with the diesel seems to be blockage of the EGR valve (exhaust gas recirculation). On our car this has caused rough idle, and generally poor acceleration and fuel economy. At idle the tacho shows the engine stumbling gently for a bit, then clearing. Theory is that it's worse in Australia than elsewhere due to poor fuel quality compared to Europe. We've had this twice in 15,000km the first time accompanied by an engine-fault message, the second time recently without any message. Both times it's required removal and cleaning of the valve, which is not a huge job.
The other common problem is the DPF (particulate filter) in the exhaust clogging, especially if you drive short distances. The DPF needs to 'regen' basically by closing a solenoid valve and running a very rich mixture, until all the soot in the filter burns away. If you don't drive a decent distance at a fairly constant speed, it can't regen. I think if the DPF cannot regen you get a specific message on the dash.
However - these are really common faults with the 2.4 JTDm, and I'd expect a genuine Alfa dealer to identify those problems immediately - so it's quite possibly something else.
Which state are you in?