vaxhall vectra b diesel cutting out??

my vectra starts everytime, and will rev fine, but when i lower the revs just about 3,000 rpm it dies and cuts out, turn ignition of the on and she will start sgain??? but if i dont give it some throttle it will cut out within a few secs. some days it will run fine and halfway through my jurney it will start cutting out and jerking, as if i was taking my foot off the throttle and on off on off, ect, any help, thanks ps, i have tried changing crank shaft sensor!! and no joy.