Well, I had guests coming Wednesday and I needed the Landy up and running - wanted to show off
So I started to do/fix pretty much everything I could think of.
I have cleaned the earth points (battery to chassis and transfer box) and just to make sure I have pulled an extra earth wire as well. I have pulled one from the battery to the ECU earth and one from the chassis to the bulkhead as well.
As I was working under the drivers seat I have disconnected the ECU and found oil in the red plug. It has pointed me to the injector harness: It becomes oily when the injector harness is getting old and not sealing properly. (Oil is seeping down in the loom along the wires.) But an old and brittle injector harness can also cause the misfiring I had. Before popping the rocker cover I have double checked my suspicion with Andy White (White's Garage Washington Lane Edinburgh) and he has confirmed that it's most probably a broken wire in the injector harness.
I have changed both (harness and gasket), sealed the loom with some extra silicone, wrapped up the earth wiring and it has solved the problem completely. I drove about 100 miles with my guests since and it's running like a dream.
It's worth to note that my temp gauge was fluctuating previously, was moving slightly up and down all the time (which is caused by bad earth connection). Now it is stable and reading as it should.
Hope this story helps