From the immense number of articles found under "iBook, back screen" on Google, I'm under the impression, that the defect is found in bad soldering of the main IC ( one inch square) to the board.
By applying pressure onto this IC with metallic a metallic insert and four plastic grommets on top of the thin metal sheetcover, the problem seems to be solved.
There is a french group which have looked into this problem, and apparently found a solution.
Look at: http://www.PowerBook-fr.com/iBook/bricolage/repair_g3_video_en_article797.html
greetings
Hans Elfelt Bonnesen,
Copenhagen, Denmark