I am with Keef on this one - typically it will be the powersupply.
The amount of dust that accrues in these units is unbelieveable - I have can of spray duster on my desk and try to give the machine a good going over as preventative maintenance evey 6-8 weeks. I still have had to change a 600w powersupply on this machine cause it went faulty - giving almost the same as what you are experiencing.