failing thermistor

Hi,
I want to ask an urgent question about a hot end thermistor, which is bothering me in these days.
Hot end was working fine. However, in these days, I am starting to heat the extruder using repetier the temperature increments are becoming very low, and after 180-190 C, it starts to show 0 C. I checked the wiring. İt seems ok. Thermistor seems working right till that temperature. At room temperature, it shows around 100k… from near the heating block or from the ramps board, no problem. The heater resistor seems ok. There may be a problem about thermistor insulation, the thermistor inside is cemented to the heater block, but there is no thin peek insulation around the wiring outside the heater block. But there is a White heat insulation (I think aramide material). I thought that since it is not a thin insulation, maybe it is causing heating at the metal wires outside the heater block, I used different techniques to isolate the metal wires besides this white insulation, I also used rock wool or kapton bant… But I could not solve the problem. I also thought maybe it is showing once 0 degrees than coming to itself and showing normal again. I disabled Min_temp so when it shows 0 C, the hot end did not trigger min_temp, but it showed zero degrees some time after that temperature started falling and it came to itself around 170 C starting to increase but again around 180 it shows 0 C. And I did not measure the exact temp with another thermocouple but I think the heater was open. The last Marlin change seemed irrevelant to me but maybe it tells something to you. 
Thermistor setting is fine it was epcos 4.7 k pullup, RAMPS board, pin connection should be right, since it was working in the beginning.
Therefore I concluded that the thermistor is failing. But since there is no problem at room temperature, I could not be so sure about it. What do you think ? Is this  a failing thermistor? Thank you.