Frans certainly knows how to think out the box. Just had a very interesting conversation with him.
One possibility for the hunting when hot and not cold as well as it only happening when in gear is a water temperature switch. He says there are 2 switches, one for the dashboard gauge and another for the ECU. If the ECU one is sending the wrong information, the ECU will adjust the amount of injected diesel incorrectly. If it thinks the engine is colder than it is, it will over oil to compensate. This fuel adjustment only happens when it is in gear and not, apparently, when in neutral. He also said that very often the sender switch breaks down at the high temperature and is fine when it is cold. And the reason why there is no engine code thrown is because the ECU accepts the information.
Unfortunately, I have to find someone with a diagnostic computer to be able to see what the readings are, so will have to go to Durban for that. Just changing the sensor wont be exact because it could also be a connector.
Will keep posting
Have Troll, will travel.