2003 Citroen C8 car missing

hi every one yet another fault with my c8 had this car about 10 week now bought from ebay big mistake when first got car shortly after started getting anti polution faults but never went in to limp mode. if it helps think the bloke i bought it off ran it on red or worse. the anti pollution fault has since gone off since i seviced it and changed all tyipical filters and had a 800 mile blast on motoway. one fault that is still there with no pattern is that it keeps missing acting like it is low on desil on certan days it drives fine but on others its a nightmare. most of the time under normal driving it is fine tend to find when you back off on the accelerater and then put your foot down its lazy and jolts any help or advice on this would be much appreciated mant thanks