2004 Daewoo Lacetti Losing Power

Ever since the car was about a year old the check engine light has been on. The SERE engineers said it was nothing to worry about at each service interval. For the last year the car has been losing power to the point it is dangerous to drive. I recently had it hooked to a computer and the only thing it says is 'multiple mis fires'. I had the leads, plugs and coil pack changed. Light stayed off for a day! It drove like new for a week, then the problem has come back. I've just spent  £250 and really puzzled. Grateful for any advice. Car only has 20000 miles on it.