Galaxy S4 Freezing, rebooting & Vibrating Twice

[Stef2017]

Hello. My dad was recently given a Samsung Galaxy S4 (GT i9500) by a friend & while it works perfectly (No lag, slowdown etc...) while it's been used; at times when its locked the phone will seem to freeze so that when he goes to unlock/use it it won't come in, Instead a few seconds after the power or menu button is pressed it will vibrate twice & reboot.

I initially through it might be a battery issue as it was charging/discharging quicker than I would expect so I brought a brand new official Samsung battery but this didn't fix the problem.

I next performed a factory reset but this again didn't fix the problem. I removed the SD card to see if that was the problem but this also didn't fix it & I also tried removing the sim-card to see if that was somehow causing an issue yet again this didn't fix it.

The phone is network unlocked but as far as i'm aware it hasn't been rooted or modded with a custom rom or anything (It's running running Android 5.0.1).

I know that given how a factory reset didn't fix it its probably a hardware issue & am just wondering if anyone has any idea on what may be the problem & if there's anything that can be done to figure it out so that we know if its a possible fix or not.