engine woes

[neb]

hi
i have a p reg, 1.0l polo (4 cylinder, 1043cc). its a pain to start when cold and is reading a hall sensor g40 fault on diagnostic, i had the distributer unit replaced but hasnt helped, after a couple of seconds idling it begins to struggle and when i try to bring the revs up it splutters and stalls. it also can stall on the overrun. it runs fine as long as i keep the revs above 1000rpm.
its recently had a full service and done 64000 miles.
any help would be much appreciated