RF based admittance type level sensor

Hello guys,
I need some clarification and guidance from this group. Here is the situation:
We are using RF based admittance type level sensor to measure the level of a material(PTA slurry). Temperature is 175Deg, Pressure is 0.8MPa. Now in admittance type level sensor, there is electrode, insulator. For our application, electrode is of SS304L and insulator is PFA. When the sensor is new, PFA insulation is tightly wound up on the electrode. But after few weeks of operation, this PFA insulation looses the grip on the electrode, leading to erroneous reading. Sometimes, the PFA insulation gets removed from the electrode.
Now, the reasons for this is not clear and i was hoping that someone could provide me some answers and possible solutions for this type of issue.