iPhone frequent random restarts

I have a iphone 6+ and since earlier this month it started to randomly restart with that apple white screen, frequently.

It's mostly switching between apps or just basically while in use. At one stage it was doing it on it's own while sitting there on recharge.

I've tried resetting all settings and it still had the same issues.

I was one step short from wiping it all clean, but funny thing, I had the exact issue with my iPad Mini back in April...I had wiped my iPad mini and with no new app installed it still had the same problem! I just never sent my ipad mini in for repair as the warranty happened to end in April exactly.

Last week I sent in my iphone to a repair centre, explain the whole story, but of course the person who took my service request just put "random restarting" in the issues column...6 days later, after I called them to enquire on the status of the repair, they said they've resolved the issue by restoring my phone to factory settings, and claimed a software issue.

So $38.50 later I brought my phone back home and within two hours, the same problem occurred again twice.

I haven't even restored my phone to my backup for gods sake. I was only using safari...

Help! What should I do next? Take it to repairs again so they can charge me another $38.50 to reset my phone???