it's difficult without getting hold of the machine
couple of things come to mind ,,,
1) 3 years old ...and you say it won't reboot immediately ... I wonder if it's overheating?
Open the box ... does it look like a sort of grey snowscene inside?
especially the fans ... if they look like dandelion clocks ... you need to gently brush all the dust out (I use wife's blusher brush) then carefully vac the gunk out of the base
if hat's not it ...
2) take notice EXACTLY what is happening at the time ... what you are doing ... and which apps are open - and how long you've been doing whatever
just before it goes black ... any messages?