phone is self powering off with a full battery

Hi I love My HTC EVO 3D for the 3D Camera and 3D Image Touch Screen ( no glasses required ) You just can't beat that.

Unfortunately My phone seems to keep powering off right after it get's to the home screen after the password has been entered.

The batteries I use ( I have several ) are all fully charged and work well.

Sometimes the phone will power off right after I power it on.

If it is a software issue I do not know how to upgrade the software if I can't keep the phone on.

What can be done?

I would rather learn to fix it on My own

I dont' want to have to pay much $

lmk pls thanks.

ROB