HP Compaq Desktop Service

This machine, when you took the side off and had a look inside in normal ambient office light looked fine. Once the picture was taken using a flash, and then zoomed in

It was quite clear that a build up of dust had started on this machine. which had been in place for just over a year in a standard office environment.

The point here, is that machines are designed to run economically nowadays and a machine running hot will potentially throttle back its CPU if it cannot cool it down to the thresholds set.