my engine light comes on now and then.

Its only when I get on the motorway or a road of similar speed

(dual carriage way etc etc) It comes on and will stay on for the duration of the drive.

If I turn the key off and on it will stay off for another 5 mins and then pop on again.

Doesnt do it at all on town driving and the car runs absoluetly fine no funny sounds and no noticeable difference in performance.

Any ideas?