To start, I know my temp gauge is wrong, it overheated in the middle. So I figure being on the 2nd line, the beggining of normal is its norm. But on occassion, the neddle goes to the 'o' in normal, back to the 1st line and so one several times in amile or two. Then it just hangs out in its norm.
My question is....think the sender is going wrong, or might the thermostat going stupid?
Thanks

Oh, when it overheated, was because the plastic tank split. If that matters to my question.

 

I'm gonna venture a WAG that you have a problem in your dash cluster, either the electronic supply regulator or a questionable ground.

Every now and then my gauges all go nuts-temp, oil go off scale, and I have a tank and half of fuel. Cycling the tank switch selector a few times usually restores reality.