Dirty diesel td5 discovery

Lately when driving at ±120 km/hour disco engine jerks as if dying and backfire. If i back of the throttle the engine going smooth i do not press the throttle. Afer a while everything going normal. It happen intermittantly . Other days no problem what so ever .
Today after start engine run for few seconds then die. Start for quite a few times the same thing happen.

Start engine run for only ±5 seconds then cut out i have to turn the key to the off position then start again it takes immediately but only run for the 5 seconds cut out same all over again.Left it for an hour took immediately and gave no problem still running.
I did take ot the diesel tank clean everything and thought the problem was fixed . Next day same problem all over again. Had to travel for 10 kms starting switching of now the Traction control and ABS lights stay on.
Any advice?
Really Stuck

Have you checked if oil has contaminated the wiring harness. Quite a common problem, causes all sorts of faults, especially misfiring and poor engine running, do a search and you'll come up with some info.