I too am having this issue, I was referred to this thread after I posted my own. Anyway I seem to recall having a problem where my Tiger install would lock up periodically, I was overclocked at the time (stable in XP) so I put it back to stock and it fixed my issues. Now after reading the replies here it makes sense that turning the HT link down or loosening up the RAM timings might in fact fix this issue. I am currently overclocked right now which hasn't made it any worse than before (different CPU than I had with Tiger). I am going to loosen the timings and I will let you know if it makes any difference.

AMD Opteron 165 (Dual Core)
DFI Ultra-D nForce4 Ultra
G.Skill 2gb RAM
eVGA Geforce 6800GS (QE/CI)
Buffalo WiFi (Broadcom chipset)
WD 120gb SATA (Leopard)
WD 250gb SATA (Time Machine)
WD 250gb SATA (Windows XP)
WD 500gb SAT