Rev problems, engine dies when vehicle stationary, low power, high fuel consumption

I have a KB 240 LE petrol engine. When changing gears and when I come to a stop sometimes the rev meter remains high though my foot is not on the acceleration pedal. If not high revs then the rev meter drops to zero and the engine just cuts off. When driving on a highway at high speeds, the car seems to be dragging, and at times it just opens up no drag anymore but only for a short while. On fuel 10 liters won't take me more than 60 km.