Hello there. I'm having some issues with my mid-2015 iMac 5K. Very recently it keeps shutting down of it's own accord. I have to unplug the iMac from the mains and wait a good few minutes before I can power it back on again. I've spoken with Apple support and tried the Diagnostics test, a SMC Reset and even different mains sockets but it still persists. I am booked in to the Genius Bar next Thursday afternoon and am hazarding a guess it's hardware related. As I'm out of warranty (it's a second hand iMac purchased via eBay and has been running fine since November 16) what kind of fees should I be looking at? Thanks.