[DaiTheDragon]

I have a G5 1.6 512 ram with osx tiger. The fans have started to increase speed and then die down again as I am using the computer. Is it because it is overheating. Should I give the inside a good hoovering (being careful of course).

Dai

[Waragainstsleep]

You need to run thermal calibration on it. Not sure if this can be done from its hardware test discs. An AASP can certainly do this for you.

[OreoCookie]

You should download a tool such as Temperature Monitor (just google for it) and check whether the temperatures (as measured by your system sensors) really spike. That should give you an indication which parts of your system need to be cleaned.

[chris v]

The fans do throttle up & down, depending on what you're doing. Mine spin up a bit when I run Photoshop filters & such. They also tend to run faster in the summer. Blowing the dust off the heatsinks is probably a good idea, though. Go get a can of compressed air (any tech shop'll have it) and take it outside. I dust mine every 6 months or so, and it's always worse than I would have thought.

[Waragainstsleep]

You might also try monitoring your CPU usage to check for any correlation. The single proc 1.6 shouldn't heat up anywhere near a dual. It could very well be a build up of dust in the case or fans.
If you have your original discs there is a hardware test on them. Worth a blast, you might be able to run thermal calibration from there too. Possibly not though.