For some unknown reason my stock temperature gauge is playing up! When I warm up the car, the gauge will normally raise above C then stay in the middle. If I turn off the car then turn it back on, the gauge will stay under C even though the engine is still warm??? This has been happening for about 1 month...

 

I would opt for an aftermarket gauge. I just put one in and wow what a difference.
The stock gauge is a pos and if something does happen and your car begins to over heat the stock gauge won't let you know until its too late

 

You could have a bad contact or loose wire from the temperature sensor. Find the temp sensor and check the connector.