DS18B20 error reading

Hi to all,
this is my first post, my name is Gaetano Pepe from Italy.
I've been using the DS18B20 sensor in data logging applications. I have a problem with the sensors: some of them, after some time (weeks, months,) of correct readings, start giving a completely wrong reading that in some case is less that 35 degrees centigrade of the real temperature.

The DS18B20 is assembled in a stainless steel probe with a 50cm of cable.

The only remedy for this problem is to replace the sensor with a new one.
To save the battery, I use to power the sensor before reading and turn it out after reading; the sample interval is usually 15 seconds.

Have someone had the same problem? Does the DS18B20 suffer of some critical issue?
Thank you in advance for any help.
Gaetano