I tried to switch it on this morning. Nothing. No noise. No fan and no power light. It's as if this Mac is not connected to a power source at all. But try to start it up a number of hours later, it works as if nothing had happened. This bizarre behaviour has happened a number of times now over the past week. (And I have reset the SMC but to no avail.) It can't be the PSU. I'd appreciate any ideas.