my C.E.L has hit the very serious condition whats the usual problem when this happens on 1997 Land Rover Discovery

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three weeks ago my C.E.L. came on,well the rover runs good till you get to normal running temp then she nose dives , wont go but maybe 20 mph back fires wont accelerate. well this week the converter turned red and started a fire on the under side and burned cables and wires. those have been replaced ,now i need to fix the original problem. what is the usual cause
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The car runs extremely rich.
The usual cause: bad fuel injector (stuck open pintle) keep dumping fuel, non-sealing valves, failed MAF sensor, failed O2 sensor, fouled spark plug in one or more cylinders, pretty much anything that could affect the correct air/fuel ratio, AKA fuel trim.

then why does it run good till it hits running temp. something has to open at temp and cause this condition IAC or EGR something that will reset once engine temp has cooled