Overheating
It's Summer in the UK and it's quite humid, therefore little heat transfer to the atmosphere from humans, and heatsinks
The SITA PC at work has been playing up lately - 1 PC stand (air underneath) and moving the PC to one side (stops books being placed against hot exhaust vent) and it starts working again (100ft of paper started printing at 4am this morning!!!!)
The PC now has a marked (with marker pen) air gap beside the PC case to prevent the machine from overheating (about 1.5")
My own PC at home has the side off permanently to aid cooling, but will still occassionally shutdown during summer months