It's the operating range of the temp sender not matching the requirements of the temp gauge. I run a 3,9 injection in my 130, I bought a matched gauge and sender as a package from Burton to guarantee the correct readings. Trying to get an accurate reading throughout after fitting a variable resistor would concern me, the original gauge fitted on my Land Rover had a resistor fitted which worked ok but I found the gauge only read so far. Unknown to me it had developed a water leak from a hose, lost a lot of water and the gauge never went above two thirds. Luckily I was stopping at services and caught it but the engine was overheating badly. I would go for the new gauge and matched sender, I have managed to keep my one to roughly 80 degrees most of the time according to my set up now.