You may have a faulty Power Button?
The only way to rule out software is that Reset you are avoiding.
BTW.... A battery pull is NOT a soft reset. Battery Pull is hard on the system. It's like stopping your car by running it into a wall. Something is likely to break even more.
To ensure the best possible outcome reflash the device so it does not repeat any errors in the image on the phone, which a factory reset or hard reset would repeat.
FAQ - How can I recover/reset/restore my phone software? - Microsoft - USA
If a clean install does not solve this, seek qualified repair or warranty replacement.