110 defender TD5 engine missing under load

Hi All
Can someone help me with my Defender 110 TD5 (1999) problem.
The Landy starts and runs perfectly except when I am either (1) pulling my trailer with a heavy load up hill (2) I am accelerating at almost maximum revs usually in the higher gears, again usually up hill she will then start hicking and farting almost like it was missing or fuel starvation.
I have changed all the fuel filter under the rear wheel, dropped the tank and cleaned it out and have fitted a new Bearmach fuel pump but all of this has still not resolved the problem.
If it was the injectors the problem would be there all of the time wouldn't it ??
I do have an engine oil light which flickers on and off but the engine has the right amount and grade of oil in it. There is a loose wire in the oil switch but even if I connected it the red light still flickers on and off.
Can any one help please
Cheers Gordon

You need to check out the oil problem asap. It may be the oil pump sprocket bolt working loose which is deadly on the TD5. Some early pre 2000 engines did not have the sprocket bolts locktited in the factory.

As for the engine being down on power does it clear if you switch the engine off then restart? If so the ecu is switching to limp mode probably because the turbo is over boosting under load at full throttle.

air flow meter.

Check for free movement of the turbo waste-gate sounds like it's seized