take the tank out, drain it, clean it out and refill with some fresh, clean fuel. sounds as though you have picked up some dodgy fuel. or the coil is playing up. also i would replace the spark plugs.
here are some pics of my mk2 cortina will try and get some of the engine bay. the car is fitted with a 2ltr pinto, but needs some fine tunning on the conversion side of things. has any body got a mk2 with a pinto fitted any advice on this conversion would be good. jon