Originally Posted by T-bucket23
You may need to drill a hole in the t-stat. If you dont have a heater internal circulation pressure can hold the stat open or closed.
The first thing I'd look at is the coolant level. If you have air trapped in the system, the thermocouple that senses the coolant temps will fluctuate considerably. It can not sense any temp in air and will yield a low temp until some coolant passed over it to then yield a higher temp.
I'd look at modifying or swapping out the thermostat second. Some new thermostats come with the relief hole already there. A 1/8" hole is sufficient.
Without the hole the gauge reading will fluctuate quite a bit until the coolant temps stabilize. The hole allows some coolant to flow thru even when the thermostat is closed or only partially open. This will show a more even temp rise without all the fluctuations.