[G4] Shut down/ restart problem

[Peter]

My G4 has recently refused to Restart or Shut Down from the Apple menu. All
the applications quit but I am left with the blue background and have to
press the power button and hold down for a few seconds until it switches off.
[the volume also sets to mute when I re-boot] I can get it to sleep using the
Apple menu or the power key on the monitor. It only appears that the Restart
and Shut Down which are the problems.

I have tried...

resting the NVRAM & PRAM -battery reads 3.7v
[cmd-opt-p-r] [cmd-opt-n-v]
reseting PMU (power management unit)
removing and re-seating the memory
removed all unnecessary peripherals
run Applejack for Tiger
run TechTool Pro v4.0.4
run DiskWarrior v3.0.3
started up in Safe mode

I _may_ have tried other things but just now can not recall.

I am unable to say when this problem was created as my machine is normally on
24/7 and did not come to light until I was forced to re-boot after installing
some software -and yes I have checked it. Could it be the upgrade from 10.4
to 10.4.1?

Any help/ suggestions much appreciated.

[System- macintosh G4; OSX 10.4.1; 1GHz; 896 Mb SDRAM]
--
-- 
Peter

The box said "Windows '98 or better", so I bought a macintosh.