You might try cleaning out the fans and air passages and stuff - sounds like it might be overheating.
What do ya know.. Turned out to be the 2 2GB sticks of PNY RAM I added when I got my G5.. Now Im back down to the lowly 2GB that came with it until I can replace it with some Crucial RAM this time!
Oh and u2 built just fine on PPC 10.5.8 yesterday.
