iPhone 5 powers off and restarts

This iPhone 5 has recently started shutting off (hard power off) and the screen goes dark sometimes with a momentary appearance of vertical lines on the screen. Then after a few seconds the apple logo appears and the phone boots up. The battery inside has been unstuck from the base for sometime, as I can feel it moving when I turn the phone around. The phone has not been opened so far, but I have ordered for a replacement battery/tools and will give it a DIY and see. On iTunes the device shows up as having the latest ios and the phone otherwise works fine, for a few seconds or minutes before powering off and restarting by itself, so I am hoping that replacing the battery will set things right.