remember that in G4 iBooks, it's not necessarily related to a faulty GPU, but rather delamination of a vreg chip on the underside of the motherboard:

http://www.xlr8yourmac.com/tips/iBookG4_vreg_repair/ibookG4_vreg_repair.html

... I'd try resoldering the legs of the chip pictures on the 933 and 1333 models and see how you go!

JB