check engine light comes on

Not all tyhe time, just at weird times while driving. Sometimes when cruising at 80-100kph, other times when sitting at the lights. I have changed the plugs and leads, not a cheap job!!!, about a year ago. Sometimes it stays on for quite a while , other times it is just a flash then its gone again.
Could it have anything to do with a clean air sensor( if there is such a thing fitted in the car), or am i just clutching at anything to blame.

The car has done about 268000kms and i changed all filters and fluids every 10 000 kms.

Any help would be fantastic, i dont want to book it in to service and pay through the nose just for them to say they dont know whats wrong with it.