how can i fix?

berlingo van 19d 2001 van ran well then developed a slight miss after about a week ,suddenly the van was missing drasticley and purging light blue smoke which got worse at 3500rpm,i removed butterfly from egr valve ,disconnected air recirculation pipe and blocked it off.it ran like a dream more power no miss etc .on fitting a new egr valve the problems developed again.the day after .