Why is my PS4 Slim Crashing During Gameplay?

My PS4 slim crashes during gameplay - the TV loses signal, and the controller won’t reconnect, but the power light on the PS4 stays on. The power button won’t respond either - I have to turn the mains on and off again before it will boot up.

After rebooting, the PS4 then crashes again, but sooner, even when there’s no game running. At first, I though it was overheating, but there wasn’t the standard error code or beeping that indicates overheating.

So far, I’ve tried:

• Unplugging it from the surge protector.
• Running digital games instead of physical (the PlayStation support guy thought it was a disc drive problem).
• Re-building the database.
• Re-initialising the PS4.
• Plugging into a different power outlet and TV.
• Replacing the internal hard drive.

None of these have made any difference. Could anyone tell me what the problem is?

Edit: To clarify, there are no error messages or anything else to help diagnose the problem.