1995 Other Rover Models please help:-)

How you doing, i'm having trouble with my car and i'm in two minds whether its the coil or the lambda sensor or a bit of both.I have replaced the ht leads, plugs, rotor arm, dizzy cap and coil and still the problem persists.I have very bad mpg.150 miles from 55 litres, the car shudders mainly in 3rd 4th and 5th, it backfires also and runs on 3 cylinders. Its fine 50 percent of the time but the other 50 its a nightmare.I ran it with the lambda unplugged and it was alright, gave only a tiny shudder from a 25 minute drive. The brand new coil isn't bolted onto the body so wont be earthed properly but would it cause all of the above problems. Will be very grateful for any advice as i'm not really getting anywhere with various garages. Thanks alot. All the best.G Fraser