how can I fix my 2004 1.0 corsa twinport?

when driving my car recently round 30mph, its started to feel slugish, however when I go any faster, eg the motorway within 5 mins of traveling 70mph the engine mangagement light comes on and car becomes even more slugish and feels like it is slowing down even with my foot fully down on the accelarator, the car remains the same even when i come off the motorway and the revs then dont go over 3. however when I switch the engine off and on again the engine mangagement light goes off and car returns to just slightly slugish. I now dont want to drive it anywhere far