My voltage and temp gauge do this, as do many others. I suspect personally it's the gauge not the sensor. One might try taking the wire for that sensor once you find it and grounding it while the gauge is "stuck" on cold. If it don't move when grounded, then you wack it and it goes full hot reading, it's not the sending unit. I plan to pull cluster apart and use a light machine oil carefully on the needle base. Though for the record I do recommend replacing both temp sensors in the head if they are twenty years old.