Computer randomly powers off

My computer will run fairly well for about 30 minutes to an hour and then it just dies. It's like a power failure, but nothing else turns off. The monitor, speakers, desk lamp, external hdd, are all plugged into the same power strip. This setup has been the same for over a year and the computer has been shutting down randomly for a couple weeks now. It has been sluggish for a few months despite cleaning the drive, defrag, etc.

I recently opened the case and gave everything a thorough cleaning and blowing out (much needed), and it started running more quietly for about a day and now the machine is getting loud.

Could this just be the power supply getting old and the dust wasn't helping matters? Short of replacing the power supply, is there anything to do to possibly prolong its life? It's 6 years old this year, and frankly, I'm surprised my computer has lasted as long as it has, with all the moves and near constant use.