Intermittent idling

I have a 156 1.9JTD M-Jet '04, its just been in for its 4 year service and its seems to have developed a habit of semi-revving when its cold and idling. It sits at about 750 then goes up to 900 then back down then up over and over. I have revved the engine and it settles then immediately starts over again...

Any ideas?

Oh God. Not yours as well! What is it with alfas, one minute they are the bees knees then it all goes pear shaped!! Drives you crackers!

I am thinking timing belt myself as that has just been changed by the 'monkey with tools' I used