I have an iMac G5 bought back in the day. It currently sits in my living room and is used mainly for the basics of email, RSS newsreader, newsgroups (yes, they still exist), browsing and remote speakers (using Airfoil). I used to use it for iTunes and iPhoto but moved them to the more powerful office machine when they started to become very frustrating to use as they became glacially slow. Despite removing them the G5 kept getting slower while the fans kept getting louder. Recently the dreaded random sleep appeared.
The first release of the G5 was not Apple's finest hour in manufacturing quality. The capacitor issue affected many makers and mine ended up with a new motherboard thanks to that. The people doing the work then managed to break the DVD drive (requiring yet another replacement) but let's not dwell on their stupidity. Earlier this week I was reading email and the machine went to sleep. Moving the mouse caused it to wake up. A minute later it went to sleep again. I checked to ensure that I didn't have any sleep corners defined in System Preferences (it went to sleep while I was doing so). A quick Google (involving three machine sleeps) indicated it was either a power supply issue but one suggested it might be related to ventilation.
I unplugged the machine, laid a towel on the bench and placed the G5 face-down on it. There are three captured screws at the bottom which allows the entire back to be removed as soon as they are unlocked. There was a small bit of dust spread around but the noticeable item was the huge furball of lint that sat near the bottom. On the G5 there is a small air vent just below where the power cord plugs in that is where a lot of the air required for cooling comes in. The mass of lint was blocking the space between the vent and the motherboard. Removing that and giving the rest a quick going over with the vacuum provided a working machine again. Using the iStat widget I can see that fans are running thousands of RPM below their previous levels and the internal temperatures are 15-20 degrees lower. The machine is responsive and usable once again.
As someone who writes software for a living this merely supports my belief that the only computer problems are hardware ones.