Phone Turning off by itself.

So I have this weird issue with my HTC ONE M7 for a few days now. It would turn off by its own when I was using it but I'd just turn it on and it will work again. I thought it may have something to do with a bad app or the rom might be acting up. Anyway I ignored it since turning it on was a minor inconvenience and I needed my phone functional. Today whilst using GPS, the phone suddenly went dead on me. Every time I boot it up, it passes the initial HTC screen but then doesn't boot into android. The screen goes black and the phone turns off by itself. I went it recovery, wiped dalvik and cache thinking it was an app causing a boot issue but that didn't help. The problem continued. I already had a backup so I went ahead and tried restoring it. It worked the first time. But the phone still won't boot. It would turn off automatically. Then I tried reflashing it with the backup. This time the phone would just turn off as soon as the restore process started. I finally got tired of it and decided to reflash with a new rom. The flash went fine. But lo and behold the same issue again. Boot animation and then black screen with the phone completely turned off.
I thought it might be the battery so I plugged it in to my laptop USB. It booted but when i was setting up android, it would just shut down again. Then I connected it to the wall socket thinking the phone will get more juice. This time around I was able to set it up. With the phone connected to the wall socket all was working splendidly. Please note that during all this time the phone was at 100% and there were no drastic falls in the battery levels.
As soon as I disconnect the phone from power, any power intensive operation shuts it down. I have been in recovery mode for 2 hours now and the battery hasn't even dropped by 3%.
So the question is, What the hell is going on? Is it the battery, is it phone's hardware or is it a software issue? Any help would be greatly appreciated. Cuz if it's something that can't be fixed, I have to start thinking about getting a new phone then. THANKS so much for bearing with me! Cheers!!