8915 keeps powering on

Why does my 8915 keep powering on? I'll start looking through the menu and it will suddenly power down and then back up again. I reset the phone, bought a new battery. Theirs no pattern or anything. The screen goes black and then it reads "power on"

I bought it over a year ago on Ebay. I went into Virgin XL and that is when it started having problems. Do you think I could have activated something?
Sounds like it may be time for a new phone.