I used to experience a temperature needle that would climb to the middle as the truck warmed up, and then randomly would drop to no reading, and then jump back to the middle... back and forth, back and forth... at times it just wouldn't work at all... I had this happen on two different trucks... conveniently, it happened after I changed the coolant in both trucks... I finally spent a BUNCH of time burping and reburping the coolant system... apparently there was still some air in the system somewhere... My problems finally fixed themselves... I think that if you have even the slightest amount of air in the system, that the air might congregate around your temperature sensor, which causes it not to work sometimes... But your problem sounds a little different...