Well, it seems the power button on my 2 year old Nexus 5 is sticking causing all sorts of misery atm. Might be time to look for a new phone
Had same problem following dropping it in the toilet
Took back off, disconnected ariels, battery, camera, etc, and took mother board off (absolute piece of cake, honestly), and then gave the power button a good hard blow. Been fine ever since.
They recommend a soak in isopropyl followed by a blasting of compressed air, rather than just blowing on it
It's also possible to buy a new power button and solder it onto the motherboard, if you've got very good soldering kit and skills.
You can also send them off to be repaired for you. A lot cheaper than buying a new phone anyway.