'36 temp gauge
 

the red fluid in my temp gauge rose to "half mast" on its own with the engine cold. and then it rose nearly to the top of the glass tube, again on its own. all this with the engine cold.
when the engine is warm, the fluid rises all the way to the top of the tube. or at least above the opening in the face of the gauge.
its original and worked like new until now. I tried putting the sender in a cup of ice water with no effect.
the sender has no dents, and the tube has no kinks.
anyone know what might have happened or if it can be repaired?