When the heat needle start moving up, then reasons can be
Fan is not turned on.
Radiator running short water.
Level of Engine oil is lower than normal
When you are driving and suddenly the heat needle start moving upwards (means showing more temperature), then I pull the car aside, let the engine running, and then throw the planty of water on its radiator. After couple of minutes the built pressure will be released, needle will come back to its normal position and then we can open the radiator cap (very carefully by putting wet cloth on it and very slowly), then can add water in it and its bottle, while engine still running.
This is what I used to do with my previous car.