Could be one of a few display support components, such as the display inverter, among others. The parts may vary somewhat depending on the build model series PowerBook G4 15" you have. There are several models listed at iFixit.com so you'd need to further detail to identify the correct one before looking serious to compare components, & methods of troubleshooting may be a little different when the parts vary. Not so much, to an expert.
There may be more than one issue behind the existing symptoms; at times, going into a portable computer to DIY fix one thing may create new problems.
In the aluminum series, there are two 1.67GHz models...
http://www.ifixit.com/Device/PowerBook_G4_Aluminum_15%22
Most portable Macs eventually may get a winking screen or fluctuation in brightness when the display is moved at different angles, as they age. Some were better made to compensate for wear, others maybe not so much. Plus components on the board that do not move, also may be acting up.
So that would a be place to look, in addition to sites such as everymac.com, and a few other specialist sites where they offer repaired older Macs, most parts, and perhaps have a complete modern service facility.
Good luck & happy computing! 🙂