Temperature gauge probs?

[Bundybusa]

I have previously always noticed my temperature sitting quite static at just below half and not moving. It now seems to reach that same level but steadily drops down to quarter then rises back up to just below half. It does this over and over, and is not just a "once off, here & there" occurence. I am wondering if my gauge may be dodgy or if the thermostat is opening and closing way too often. Any ideas or similar experiences?