Land Rover Repair: Engine Discovery 3.9L unstable, coolant temp, workshop manual

Question
Dear john, I have disco 1996 3.9L.

Engine unstable, if you look RPM will going up and down.and if I  enter D(matic) , RPM will going down until dead silent engine.
And the smoke is black.spark plug use champion, but spark plug good working  only 2 days and then all spark plug dead. i have to change with new plug.
airflow,TPS had cleaned and function ok,

Could you help to solve this problem?

Answer
I'd check the fuel pressure.  It should be holding about 40psi.  Next I would check the coolant temp sensor resistance and compare to a chart for the temperature at which you take the reading.  In other words, if you measure at 22C, look in the workshop manual, and see what your sensor should read at 22C.  Those seem to me like two likely possibilities