2000 Mitsubishi Pajero power

Once engine heats up the car refuses to rev. The fuel consumption indicator goes crazy and measures 2.2 kilometer per litre at 60 k/hr.
On surfaced road it will rev up to about 2000/2500 but on rough roads it will get weaker and eventually only idle.
Stop for 5 minutes and car is okay again. Full revs. Few kilometers driving and problem reappears. We added an extra fuel tank last year August. Seems some dirt were introduced and the in-tank filter was clogged. We replaced filter but problem remained. Realised micro filters in high pressure pump were clogged and cleaned them. By then dirt got into injectors so the engine had a bad missfire.
Had injectors serviced, new plugs, new plug leads, missfire is gone but engine still looses power after driving a while.
Tests shows no error code on fuel pumps but somewhere on air flow sensors. Sensors and valve body were cleaned etc.
Mechanics cannot say for sure that problem is air flow sensor. Sensor is reasonably expensive locally so I would not like to replace on some vague possibillity.
When the problem appears it seems like the car engine gets too little air or fuel as it just seems to be starved/smothered. Thus airflow can be a problem. I do however not understand why the fuel consumption indicator goes mad. Why is the problem gone for a very short while if the engine is switched off for a few minutes.
Why is the problem worse on rough roads?
The fuel pressure from the in-tank pump seems to be okay (3.5 Bar).
The pressure sensor in the common rail does not register any error code.
If the problem is a fuel starvation, why does it show an increase fuel usage?
If no one has any ideas, are there at least someone who can please tell me where to find the sensor that registers the fuel consumption or at least the flow volume so I can check that?