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.