55 Focus 2.0 Temp Gauge Issue?

[sevasa]

Hoping someone can help

I have a 55 plate focus. 2.0

recently my temp gauge has starting dancing. I have no issue when I first set of but once the needle sits at half way (80/100) it starts to fall down to cold, the shoot back up half way then down to cold etc. I have encloced a video on youtube of it doing so.... http://youtu.be/BAsxdp9Fte0

Is this my thermostat sticking or a broking casing maybe?

I am not losing any coolant and the needle never goes into over heating.

I'm lost?

[jeebowhite]

Start with the simple, its normally an issue with the expansion tank cap that might have a light split that opens under pressure.

Seen this recently on a few cars.