2002 Vauxhall Vectra engine management light

hi wondering if you can help....i have a vectra,engine management light came on was put on computer sayin misfire on block1.changed spark plug,was ok for about a week then started playin up again,so then we changed ignition coil pck and new set of spark plugsx4.wen changed started car and had big ploom of smoke then cleared itself,after a week light has come on again then off then come back on,now cleared codes waitin to see if it comes on again,any advice on what else it could be.....thanks paul