On several occasions over recent days, I've put my G5 (dual 2Gig, running OS 10.5.4) to sleep, gone off to do something for an hour or so and returned to find it running with the fan full on. It appears to have woken from sleep, but there's nothing on screen and it doesn't respond to the mouse or keyboard.
I can re-start it by pressing the front-panel button for a few seconds - and the computer appears normal after it's re-booted - but the same thing happens again a couple of days later.
It's really quite worrying as the fan is running at absolute maximum (you can hear it in the next room!) and obviously wouldn't stand up to this for any length of time, so I've started turning the computer off completely before leaving the house.
Could anyone suggest what the problem might be?