Just a guess, and I'm sure Alpha will be able to say whether this is a factor, but if you haven't done a reinstall in a while, or haven't run somekind of disc utilities to defrag your HD etc, it could be that the machine is working harder than it really needs too........ As I said I'm not sure if something like that would really contribute.... but it was just a thought......
with my TiBook, when I'm doing extensive rendering and especially now it's summer, my office gets hotter and hotter (no A/C) I have a little desktop fan that I direct at my TiBook to keep it cool........ works a treat....... 😀