CPW no help! Samsung S6 Freezing and Restarting Issues

trills

I've been having an issue for months with my Galaxy S6 phone. It started with the phone restarting on it's own now and again, every few days, and then also started freezing when idle or in use. When it froze, it wouldn't let me turn it off. At first it was only intermittent so didn't interfere with the phone use heavily, but it got progressively more and more frequent, sometimes restarting or freezing as soon as it had restarted, eventually refusing to even restart (just black screen).

I tried the following things individually and tested the phone after each, but the issues persisted:
Android software updates
Updated all my apps
Ran the phone in safe mode
Removed all my apps
Factory reset
Turning off WiFi, only using mobile data, and vice versa
I only run a few user installed apps, Facebook, Snapchat, Instagram etc, and have tried uninstalling these in turn too

My contract is with EE, the upgrade to this phone was arranged through Carphone Warehouse. I haven't contacted EE yet. I've sent the phone in for repair three times now:

Repair 1. Carphone Warehouse
They replaced 'Assy sub pba usb pcb blue', 'gold smg920f'. Same issues recurred.

Repair 2. Carphone Warehouse
They replaced '<lab location lg1g0003> tape back glass kit', '<lab location 1a0001 >screw rear case'. Same issues recurred.

Repair 3. Samsung
Their team said they couldn't replicate the issue. They took details of all the above on the phone and sent it to another team in their company (and more details, like confirming I only use original charger etc). The phone was returned to me a day later, they'd done nothing but remove the screen protector. Same issues recurred within five minutes when I received it back.

I've been using my SIM in an old and slow S4 with the same Samsung account, identical apps etc, and have had zero issues with this.

I have no idea what to try next, both companies are claiming no issue exists and the phone is unusable despite supposedly being repaired and still under warranty. I've shown them the issue occurring in the Carphone Warehouse shop each time (they couldn't even get it to stay on long enough to run their PC diagnostic tests that they have to run before sending it away for repair!) If any one has any ideas on how to fix it myself or get them to fix it, that would be great!