You could try the FiatECUScan forum see if they can help.
I am having a similar problem on a Marea TD 100 - on mine the effect appears mainly at around 1200RPM at light load and only when the engine has had time to warm up fully. After motoring several 100 km on the Autobahn the effect disappears only to reappear on the following day...Hello,
I am currently having some issues of acceleration with my Marea (the JTD130 2.4 model). A diagnostic check by my local Fiat dealer narrowed the problem to the infamous MAF sensor which I have now replaced.
The original problem involved an intermittent loss of power. With the new MAF fitted, the car produces a little more power but still has the intermittent power loss.
Any advice would be gratefully received.
Regards
Ian