Why does my Mac Pro running Snow leopard freeze on blue screen on shutdown? Symptoms


Mac Pro freezes on blue screen on shutdown. Restart seems to work quite happily, it's just shutting down that my Mac Pro doesn't want to do.
The chime has decided to become super quiet of it's own accord.
Running Disk Utility often tells me that I have

Invalid Volume File Count,
Invalid Volume Directory Count,
Invalid Volume Block Count.


or some such similar on a fairly regular basis.


Attempts


I have tried replacing the main system drive in case it was the HD that was the problem. 
At the same time I installed 12GB of brand new RAM.

It worked fine for a week and now the problem is back again.
Nine times out of ten I have to force shut down with the power button or tug the mains cable out of the back.

This was on both the original HD and the brand new one that I've just installed.


Any ideas?
Any suggestions?

I'm thinking of doing a clean install but I don't want to have to go through the nightmare of finding and running all of the installers again. Would Migration Assistant be the thing to use to help with this?

System


2.66 GHz Quad-Core Intel Xeon running OS X 10.6.8
Seems like the problem persists even after many trails of fixing it on different levels. The title is about the problem and the rest is just details of many possible solution that didn't work.

I'd suggest looking into the system.log file to start with. Specially before the last boot-up and after the last freeze points.

If nothing is a suspect of causing the problem in the system.log, then it might be some third-party extension/daemon that is the cause of the problem.