Flicking temp gauge, loss of power but not overheating??

My Defender ('03 double cab TD5) is playing up a bit? Over winter (live ooop North of Scotland) I put the usual bit of cardboard in front of radiator and side it's been 'effin cold all was well (until this week anyways). However, have noticed that after it is warm, and usually up a slight hill, the temp gauge 'flicks to the red' and then approx 30secs later drops back down to normal temp instantly - at the same time the engine kind of 'lurches' as if you've changed down too early and loses power. It doesn't seem to perform as if it was overheating, so I am just wondering if anyone has any ideas? Any help much appreciated....... PS. cardboard removed last week but problem still persists?

Possible earthing problems? Check usual culprits, battery, engine and ECU earth.

is coolant system full

Went to check coolant level just after posting.... it was down a few litres..!!! Topped up so will keep an eye on things over coming week and let you know.

Thanks for prompt responses guys!

Just took it for a thrash in the heat and up a couple of hills - looks like 'topping up' coolant did the trick - who'd have thought it eh! Doh...!! (and to think I 'used' to be a mechanic.....!!!!)

its not uncommon ,something i often see ,td5s do seem to loose a little coolant over time allways worth checking regulary

Thanks for your help James... and Derek