Screen goes black after opeing a app then comes back on 2secs later

Hi,

I have got a iphone 7, the phone seems to work fine but when opening some apps the screen goes black. the screen come back on then on the lock screen. can then open the phone and use the app as normal. close the app down open another one and same thing happens. I have tried a forced restart with the same issue. i then reset to factory settings and have set the phone up again. the same issue persists all the time. the power button does feel a bit stiff, and dosnt seem to ‘click’. could it be the power button causing the issue? or something else?