This has been an ongoing problem with my 96 Disco. I've changed the plugs, new Magnecor wires, Seafoam induction cleaning, Seafoam in the tank, changed the front O2 sensors (haven't replaced rear two yet). At Autozone they identified the following codes P1316, 0300, 0303. Car is still misfiring at 50 to 60 MPH range and failed emission test. I'm planning on adding a can of 44K to the gas tank. Any ideas as to what's next?

 

Pull a couple of plugs, including #3 and compare them for color and build up, then get back to me.

 

I would try changing your fuel pressure regulator... i had the same problems, but after changing the regulator my disco is back to normal!

 

Don't even know what a fuel pressure regulator is. Can you tell where its located. Is this something I can DIY. Thanks.