|Before you decide whether to go ahead with the repair you could try a couple of things:|
Firstly (probably far too hopeful) but unplug the power then hold the power off/on button in for about 20 seconds and see if it's any better after that.
Secondly, I've had random issues caused by oxide on RAM edge connectors. If you are used to looking inside the case you could pull out the RAM, clean the edge connectors with a pencil eraser then pop the stick(s) in and out a few times to clear any oxide off the sockets.
You could check any card edge connectors including SATA power connectors to the HD (if applicable). Check all connections are firmly in place and that the vents, CPU heat sink, fans are not chogged up with dust.
If it doesn't help need to think again about whether you have a hardware or software fault. From what has gone on in the past software seems likely but we can't be absolutely certain.
Always pop back and let us know the outcome - thanks