The thermistors might be not be failing so much as the wiring which is quite fragile, just breaks after being handled a few times. I broke the fine wire coming from both my thermistors last night. The black section of wire protruding directly from the thermistor is the most fragile part and won't tolerate too much bending and movement.
You may be able to fix the broken wire if you try.
I did successfully fix one of them by drilling into the black glue at the back of the thermistor, right next to the wire with a fine pin drill, I then pushed a fine wire leg from a new resistor into the hole and gently soldered it in with a tiny bit of solder, then cut the wire to length. I then stripped the broken wire a bit and resoldered it back to the new piece of wire. It now works again!