[Ti] TiBook Overheating

[Rafe Day]

I have an 800MHz PPC G4 1GB RAM TiBook that has started to overheat in
recent months.  After a few minutes of use, all of the fans kick on
one-by-one and it gets very hot in the center back of the unit, just above
the number lock key.  Heat is localized to this spot and it cools down after
a few minutes of rest.  Functionality is otherwise unimpaired.  I have a lot
of spare parts around, what do you suggest I replace?

All ideas are appreciated.  Many thanks for your help.