VW Polo wardning lights

My Polo has a very tempermental 'emission warning light'/'engine management light. I have replaced plug leads twice (once by VW garage and another local garage). Replaced oil and air filters had an oil change. The light sometimes flashes, sometimes on continuous and sometimes it doesn't come on at all. If I go over a pump or brake it doesn't seem to bring it on, so I don't think its a loose wire. Damp or dry wheather doesn't seem to effect it either way. I've tried, when going down hill, taking it out of gear, not accelerating and if it hasn't been on it will come on (again not always), either flashing or continuous. The cost is slowly mounting up. Is anyone else have this problem? Is it a sensor or computer problem? or is it as serous as the car manual suggests. Help please.