I had the same problem. Was working fine until the update.
To fix it I had to reset the phone: Settings/Backup & reset. Make sure all of your data is backed up before you do it. After the reset it is working fine.
I considered running it flat & see if it would work on restart but if it didn't then it would be a lot harder to reset - manually through pc.
Hope this helps.
Update
I changed my case during this process & have just changed it back. Same symptoms started happening - it seems that the power button on the case is placing too much pressure on the switch & by picking up or moving the phone it is sometimes enough to activate the button. Changing the case back & everything works fine again.
My original case was fine. The problem one was the asus flip cover case. Looking inside at the buttons I noticed that the materials were slightly different.
I would suggest trying your phone without any case & see if you still get problems.