1996 Land Rover Discovery Random Misfire Detected

Hard cranckIng when engIne Is cold most often & the sound goes up & down when throttled. Even when driving, it moves fast, slow down as if the engine has stopped running. On accelerating, it comes back to life & some times goes off which I had to recranck. This truck consumes lot of fuel.I can't use 25-30Ltr of gasoline for 45km journey. Although, the Air-Flow meter was bad Which I've changed but the problem persists beyond the previous. It produces very thick black smoke accompanied with occasional blue smoke that irritate the eyes. Pls. What do I do to bring this truck back to It normal functIonal & drIvIng life? Thanks.