AUIII temperature gauge

[Brut]

I have an 02 AUIII XR6 which has recently been serviced. As part of the service they removed the instrument cluster behind the dash to replace a blown bulb. I'm not sure if this has contributed to the situation below.

Having got the car back I noticed that the temperature gauge would climb rapidly into the red when the car is put under load ie. 115km going up a moderate hill or 140km when passing. The gauge would then return to normal within seconds of me reducing power. My mechanic first said it was the radiator and replaced it. This didn't solve the problem, then he replaced the thermostat. Again this didn't solve the problem. They have now told me that the car is not over heating and it is likely to be electronic. This is something I raised at the start with them. Has anyone had a similar experence or may know what the problem may be. Also can anyone tell me what my rights might be with respest to paying for what appears to be unnecessary work to date.