2004 Citroen C2 engine hunting

My car had lit the engine management light 2 weeks ago and was hunting it gets up to approx 2000 revs and drips down to below 1000 and back up, I have already removed and cleaned the throttle body and air filter box and pipes last weekend, this seemed to work as the management light went out too, but it's doing it again ?? What else could it be, it seems to happen when fuel is below a third of a tank but this might just be coincidence ? Really need to get to the bottom of it and as cheaply as possible please help