Random freezes and restarts with artefacts
Back in May 2017 I bought an iPhone SE second hand. When I bought it I was aware that the phone would restart randomly, but I was confident that I could fix the issue myself. Back then I already succeeded by updating iOS to the latest version and was fine until about 2 weeks ago (Sep 2017) when the problem with the random restarts re-occured.
The phone would randomly restart and sometimes show artefacts (fine coloured lines) while doing so. I have already changed the components on top of the screen (the flex containing camera, proximity sensor and such), as well as the battery, but it keeps playing up sometimes and keeps surprising me with funny restarts. It's impossible to find a pattern that causes it (wobbling flexcables or checking connections won't have any difference).
What would your recommendation be for that? Any experience? Any parts that might seem suspicious?