Temperature gauge reading high

[Devaughan]

Hi,

I have a 2000 GMC Sierra with a 5.3L. Last year the temperature gauge on the truck started to read a high temperature, I changed the thermostat and topped up coolant and never had the problem again. Until recently, the gauge started to read high again (not overheating though). I'm finding that at low speeds (40km/h and below) the temperature reads normal. As I got higher (up to 90km/h) the temperature starts to raise from 95-115. then cools right down when I come to a complete stop.

 

Now the weird part of it is that whn I go on the highway the temperature will rise as i accelerate onto the highway, then after a couple minutes over 100km/h, the temperature falls right back down to normal. Then when I come off the highway, slowing down to anything under 80km/h the temperature rises again.

 

Why could this be?

Thermostat?

Waterpump?

Coolant?

Cursed? I'm lost..

[00Silverado4x4]

Just from what I've read on here but could be the gauge itself, the gauges in the cluster use stepper motors that have been known to fail.