Why my phone screen turns off and on while using it?

[AC Question]

I have been using this phone for 20 months. So, I have been having this problem for a while. The problem is that sometimes, when I use(reading book, chatting or watching a movie) my HTC Desire Eye's screen turns off(lock mode) and then immediately turns on just like a blink of an eye. When it starts happening the screen may turn off and on several times with variable intervals.