Why does my iPhone 6+ keep restarting itself?

[iMore Question]

I recently bought an iPhone 6 plus from eBay; I got an amazing deal on the device. There's only three problems with the device:

1. The phone randomly restarts itself out of the blue. (Main problem)

2. When I enable a pass-code lock on the phone it freezes as soon as I enter the entire code on the lock screen then restarts. (Works perfectly fine until the phone is restarted) Sometimes if I wait a moment before entering the pass-code this doesn't happen. When pass-code lock is disabled this problem does not exist at all. (Mid-importance)

3. The touch ID does not work. (Not important)

I've tried everything from hard resets to multiple DFU restores to fix these problems, however I haven't had much success at eliminating the issues. I would like to know you guy's think might be causing these issues. I'm thinking there might be a possible hardware problem with the battery but I'm not sure.
(I would like to keep the phone if possible.)

 

[Just_Me_D]

If I had to guess, I'd say the either the device had water damages or the previous owner attempted to repair the device and mucked something up. Take it to a nearby Apple Store to have them diagnose the problem for you.