Download and install iStatpro from Apple's website to monitor the problem:
Consider using something like the alpha-omega cooling pad:
Other things to consider:
1) Are you placing the PowerBook on a blanket of pillow? Not recommended--the aluminum case bottom is part of the heat dissipation system.
2) There is a long, narrow hot air aexhaust vent under the hinge in the back. It must not be blocked
3) Use your Activity Monitor (AM) utility to check for runaway background processes. To see them you will need to set AM's "Show" menu to "All processes." It defaults to show only user processes.
Another issue came to light concerning the amount of heat generated inside the PowerBook G4 by its internal processors. The fan included does not seem to dissipate much of the heat. This, some critics contend, can cause issues with the logic board functionality, and even in some cases cause the hard drive to crash. Compared to the Titanium PowerBook G4, however, current PowerBook models run cooler and with less fan noise.
It is not uncommon for the 15" PowerBook G4 (2004–2005) to reach temperatures as high as 63 °C (145 °F) during periods of extensive use and operations that require a large amount of processor power, such as video editing, games, etc. Similarly, a 12" PowerBook may reach up to 66 °C (150 °F), while under a heavy load. 50 °C (120 °F) is the normal running temperature for most moderate levels of activity. A fix for the overheating is to lift the laptop above a surface.