you should make sure that you actually have a relay and not a thermal circuit breaker.
The relay will be bigger with a minimum of four contacts, two that turn it on or off and two that supply voltage when on.
Okay, if you have a relay ( unless there is some problem with you wiring ) it is either on or off. The coil that turns it on, usually is the same resistance.
Given that, the sensor in the block tells the relay to either turn on or turn off.
From what you are saying, you could have a couple of problems that would cause this ( minus the relay coming apart inside ): 1 the connection to the block may not be as good as it used to be. The sensor may be using the threads as the ground connection. If oxidation increases the resistance, the turn on point will change. 2. The wire connection to the sensor may be loose or "not as good as it used to be".
Try these first.
