After 5 years I think a bit of a service is called for, this will involve removing certain components and re-seating them.
I would imagine a fair amount of dust is lurking so a soft clean brush, I use a reasonable quality paintbrush which has been thoroughly worked to remove any loose hairs, a vacuum cleaner is useful here also. making sure you give all fans,including the one in the power supply a thorough clean.
I would remove the heat sink from the CPU and clean off the old thermal paste, I use a dab of aftershave on a clean lint free cloth, then apply a very thin layer of the thermal paste.
I would also check that all cables are firmly seated, remove and re-seat if unsure.
Post back if you still have your problem.