I noticed to day my water temp guage started to bounce up and down, from notch to another. Coolants fine, its running fine, is it time for a new gauge?
Can be caused by a ground issue. Mine started doing that actually after I installed 3G alternator. I upgraded my engine block to chassis ground to 4Ga and problem solved. Headlights got brighter as well.
Maybe not your issue but it was mine, of course there are a gazillion grounds, could be one in guage cluster as well.
And don't forget the sender in the intake manifold (I think that is where it is) for the guage as a possible culprit.