Just an aside, my trusty Mac Pro 2008 tower started behaving erratically a few weeks ago. Basically it wouldn’t power on, instead it would emit two faint relay type clicks a few seconds after pressing the power button. Leaving it unplugged overnight would occasionally result in it being boot-able the next morning.
Now I’ve been through a few spare parts on this particular computer – in particular graphics cards (it’s on its third). The overnight powerdown fix pointed me towards SMC problems, but replacing the motherboard battery didn’t resolve anything. Which lead me to the PSU. I tried using compressed air to blow the dust out of it in case it was some kind of thermal issue to no avail.