engine temp fluctuating radiclly

[Hitchedup]

Mine has done it for 16 years and I found somewhere that all thermostatic engines do it also, just most have a built in resistor(or something) to make them appear stable. These trucks show imediate temps. On older gas motors with thermocouple temp gauges they used to do the same when the thermostat opened. I look for it as a sign of normality.
But I'm new to this stuff.

 

[Ether Man]

ok guys my 97 1/2 ton conversion is doing the same thing but my gauge in the truck is running much hotter. my gauge in the truck shows about 230-240 running down the road and then when you stop or put it in park it drops to about 190. if you start reving it up the temp will begin to climb to that 230-240 mark. i have shot it with a laser temp gauge and the hottest i am getting at the back of the block is 200. do you thing i have a sensor problem, water pump, or maybe the gauge in the dash? i have installed a new thermostat when i did the conversion. thanks for any help.