[Uwon]

Presume that you are not getting any error codes.
My first guess, low coolant or air pocket in system. I would elevate the rear slightly, open the coolant reservoir, let the motor cool down at least 12 hours overnight. You may have to add some coolant in the morning before closing up the reservoir and starting the engine.
Second guess is a sticky needle. Tap it and see if you get any reaction.
Third guess is a bad or dirty sensor. When was the cooling system last flushed?
Others will, I'm sure, weigh in.
Johan