Hi, this is a long standing problem for diesels registered from 2006 onwards.
Manufacturers tried to prove that their diesel engines were the cleanest by putting Diesel Particle Filters in the exhaust system. This captures soot particles and every 15000 Km the ECU tells the fuel system to pump more fuel into the engine to burn off the soot (re-generation), the flaw in this idea is that it only re-generates on high speed journeys so if you are doing the usual commute it won't re-generate, when it clogs u,p the car will go into limp mode. Take it to an Alfa dealer and they will plug their diagnostic machine in and they will programme the car to simulate a blast down the motorway ie take the engine to high revs until enough soot has burnt off. Sadly, eventually the DPF won't re-generate and the Alfa dealer will try to sell you a new DPF £600+. A better fix is to take it to an exhaust specialist have the offending item removed stripped out and put it back in, and have the ECU re-mapped so the engine doesn't try to regenerate (waste of fuel), the results will astound you at least 10bhp more and increased mpg. Please take caution, some places don't check for faults before re-mapping, this can result in problems later. Motormech Birmingham (Italian car specialist) sorted mine and many others.