Defender TD5 rough running at low speed cured by switching off and on

Hi all,
Yesterday I drove my Defender TD5 non-stop for 300 miles, 5 hours. Amazingly, we hit no traffic problems so I did not come out of 4th/5th gear for the entire trip. As usual the Defender performed faultlessly, cruising at 65-75mph.
However, coming off the motorway near home, the engine sounded really rough, sounding like it was running on 4 cylinders and needing increased revs.
At home I switched the engine off and had a quick check, nothing leaking, engine normal temperature etc. Couple of minutes later, I re-started and it was running perfectly again.
Previously, it would occasionally run rough (from hot or cold) and turning off/on would cure this also cure it. Rarely (only 3 times in 3 years), the engine has died without warning but restarted without a problem.
Trying to find the cause, it seems it could be oil in the oil in the ECU wiring (no, it's clean) or the injector harness breaking down. Before I replace the injector harness, is this the most likely cause or could there be something else? I was worried that the ECU might be about to fail?
(- for info, its a 1999 Defender 110 TD5, 250k miles, new cyclinder head 2013, well serviced)

Try the easy thing first and change the injector harness. It only costs £25 (l bought a Britpart one and in the box was an OE part)
and takes half an hour to change.

plug it in and read any faults

Thanks for the advice, I will change the injector harness and see if it helps

Give the main ECU earth point a good clean and tighten.