Galaxy S5 :: Phone Starts Randomly Rebooting

the phone maybe 6/7 weeks ago, plugged into laptop, checked up to date, restored some data from S3 backup, installed a few standard apps, all good.. After two weeks or so phone starts randomly rebooting - not with any specific app, could even be sat on the table and you'd suddenly hear the start up noise. Spoke to EE Support, tried some 'resetting the network' thing from their end, no change. Called again and was passed to Samsung Support - talked me through a hard reset. No change. Posted back to Samsung. Came back a few days later saying no fault found but they wiped it, updated it and it's good to go. Booted and phone was crashing every time I opened Internet and a few other apps. Didn't get as far as installing apps or backup. Posted back again. Phone came back again, no fault found but same problems straight out of the box. Called and explained again, posted again, this time they've told me that the phone has been damaged and is beyond repair and included a screenshot of a screen saying 'Know Warranty VOID 0x1'.

After having a rant about their packaging and that it must have been knocked in their support centre (yes, that is my level of knowledge of phone rooting) I've just Googled and found it actually means that the phone has been rooted. The phone definitely hasn't been rooted by me on purpose and the only update I've run is through Kies. The only apps installed via Play Store. Is it possible that something that Samsung Support has done or asked me to do has changed this flag?