Tractor Repair: Engine running rough, diesel engine, fuel additive

Question
I have a JD 670 with a 18hp diesel engine.  Lately, it starts up and runs fine while mowing the yard.  After about 40 minutes or so, it seems like it starts to starve for fuel and the governor is messing with the engine (1/2 tank of fuel still remained).  I drained the fuel and changed the filter, still no luck.  Anyone have any ideas?  Thanks.

Answer
Engine running rough Question: I have a JD 670 with a 18hp diesel engine.  Lately, it starts up and runs fine while mowing the yard.  After about 40 minutes or so, it seems like it starts to starve for fuel and the governor is messing with the engine (1/2 tank of fuel still remained).  I drained the fuel and changed the filter, still no luck.  Anyone have any ideas?  Thanks.
Hi Rich.
You were on the right track with checking the fuel etc.
But was the fuel actually running through the pipe ok from tank to filter?
Have you taken out the fuel tap in the tank was there rubbish in the bottom of the tank its self? Sometimes sludge collects in the tank derived from condensation in the tank it looks like brown jelly like substance that breaks up if you rub it in the fingers but it can stop the engine as I have found to my cost! If this is the case your local auto shop should be able to supply a fuel additive.
Also after about 40 minutes the engine is getting nice and hot I wonder if it’s over heating this could cause erratic running check this out.
Martin