An accurate temperature gauge will always fluctuate because the temperature in the engine fluctuates.
Usually the only time this is really noticeable is when the engine has been working hard and suddenly has to wait at a junction or similar and the gauge can rise considerably.
You didn't mention by how much the gauge fluctuates? As long as it remains approximately in the normal zone it is acceptable but if it climbs into the hot zone or falls into the cold zone it is not acceptable and the cause should be investigated as soon as possible.
Answers & Comments
An accurate temperature gauge will always fluctuate because the temperature in the engine fluctuates.
Usually the only time this is really noticeable is when the engine has been working hard and suddenly has to wait at a junction or similar and the gauge can rise considerably.
You didn't mention by how much the gauge fluctuates?
As long as it remains approximately in the normal zone it is acceptable but if it climbs into the hot zone or falls into the cold zone it is not acceptable and the cause should be investigated as soon as possible.
Thermostat possibly installed incorrectly or stuck open. Please read more details in link.
Check the water