Thermistor keeps going out

Hi, I have been having a major problem with my 3d printer. it will print just fine for the first 5 hours or for multiple quick prints but when i want to do a long 15 hour 20 hour print the thermistor goes out so it can read temp. properly and eventually cools down so it cant be extruded. do you have any advice as to what I could do to make it last longer or to keep them from shorting out so they actually work? is there any thing I can wrap around the wires to make them last longer or one in particular you recommend? any advice would be greatly appreciated  because i am still pretty new to 3d printing thanks.