unable to halt regularly

[William XWL <william.xwl@gmail.com>]

Hi, there~

I met a weired problem, sometimes i can't halt system regularly !  When
i type `halt', it prints some message: The system's going to shut
down... and then stuck still, keep beeping ! The system is unaccessable
either locally or remotely, the only way to solve the problem is press
Ctrl+Cmd+Power to restart, and just after restart, type `halt', it'll
halt normally. Hence, seems if i run the system for some time, i can't
halt it regularly. 

I just reinstalled the system a few days ago(I'd used kernel 2.6.9 for
eight months happily, but crashed some day ago :(, here are some info:

$ uname -a
Linux williamxwl 2.6.12 #1 Sun Aug 14 11:16:19 CST 2005 ppc GNU/Linux

ibook, G4, 1G, 12". 

Well, this problem really drives me crazy ! Any ideas?

-- 
William