hello i have just brought a mk1 golf 1.1 and at first engine was tikking over high since driving it a few weeks its gradually got to the point where max speed is 40ph.i can drive everywhere mint if i just rest my foot on accelorator, but if i put it down it just splutters n missfires and dont move. i have changed petrol pump cleaned spark plugs, put red X in tank still no joy.any clue?