I have had this happen on my 2004 td5 Defender three or four times, same scenario, floored the accelerator on the motorway the temperature gauge shoots up instantly, (a lot quicker than something the size of an engine can gain heat) and as it's doing this the engine loses power, then as quickly as it started the gauge goes back to normal and power is restored, no engine management light comes.
Since I cleaned the injector harness, this hasn't happened again, it was just the temperature gauge thing that was unusual so I am pretty certain that this has to be a short circuit somewhere.
Any thoughts?