Temperature Gauge

Anyone have any ideas on what would cause the temperature gauge to fluctuate.Sometimes it stays around the usual 95 but then for no apparent reason drops back to around 40 with a corresponding decrease in the amount of heat coming from the vents. Its seems to be quite erratic.
I had the thermostat changed but the gauge is still fluctuating.
The engine is running smooth and there are no leaks in the coolant system.
thanks.

sure it's a cavy? I have never seen or even heard of a cavy other than your that runs at 95deg. and then 40deg.

Could be degrees centigrade... right? Anway from the info on other websites it seems to be a problem with one of the sensors.