xperia z 4.4.4 keeps shutting down

Hi there, i'm using a xperia z c6603 4.4.4 for about 10months. Suddenly since last week i'm encountering a problem with my device which is my phone turns off itself while i'm talking and it restarts itself while i put it in my pocket. Otherwise my device is fine to use. I think it's a problem caused by my phone's proximinity sensor. What shall i do now? Hope for help soon, tq.

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