You may want to try reseating your RAM modules. Random hangs and reboots are almost always RAM/Heat/CPU related and CPU's almost never go faulty.

You can also try a trick that I have been using for years to breathe new life into old machines that are playing up. Use a pencil eraser to carefully clean the edge connectors on all your RAM modules and then reinsert them carefully but firmly. As machines get older grime sometimes starts to form on edge connectors such as the RAM modules and PCI cards and the like. Using the eraser will get rid of it and ensure good connectivty again. This works a treat on old PCI cards that I can't bring myself to turf because they are worth a fortune, like SCSI RAID adapters etc.

I have a graphics card that requires a clean every 6 months or so otherwise one day I just get no picture and error beeps from the machine.

Good luck.