[Q] Reboot Problem topic

Hey guys,

I have bought my xperia Z3 about 3 weeks ago and I've had a few problems with it recently
A week ago, the system on my Z3 crashed and was stuck in a bootloop eventhough it was on stock firmware with a locked bootloader.
I have fixed this problem by factory resetting my Z3 with Sony PC Companion.

However, ever since that reset, it seems that my Z3 reboots itself randomly. It reboots sometimes while I'm using the phone, it almost always reboots itself while I'm asleep.

I was wondering about how I could fix this problem without resetting my phone again. Would this have anything to do with the bootloop problem my phone had recently? If so, would be a hardware problem?

Thanks guys :D