Shutdown = Restart in Snow Leopard

I have an incredibly annoying problem, mainly on my White Macbook 13", but also sometimes on my main iMac (Aluminium 2011, I think) both running Snow Leopard, in that when issuing a Shutdown command from the main login screen, the machine will not actually shutdown, but restart. This will also happen, but less often when a Shutdown is issued from the Apple Menu and it's even happened once or twice when issued in Terminal (for goodness sakes' - the *command* line Apple - how can you get *that* wrong?).
 
It's most frequent, & annoying manifestation is when I hook into my Macbook from my main Mac via MobileMe to shut it down. I hit Shutdown and at least half the time it will Restart. But, I don't have to use MobileMe to make it happen. It will also restart if I'm actually on the Macbook's login screen and issue the command there.
 
What is going on? This is a basic, fundamental system operation that a user wants (& expects) the system to *obey* - no ifs & no buts about it. Because this has happened on both my machines, I'm posting it in Mac OSX. It didn't happen on Leopard, I'm sure.

GAM
iamgamAsked:
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LeeTutorretiredCommented:
Here's a Mac forum discussing that problem with a user's computer; it might give you a few ideas to try:

http://www.mac-forums.com/forums/apple-desktops/200416-restarts-shutdown.html
LeeTutorretiredCommented:
Another forum has this as a solution:

Here's what to do to correct :

Erase those 2 files : "com.apple.AutoWake.plist" and "com.apple.PowerManagement.plist" in the Directory:

/Library/Preferences/SystemConfiguration/

While done, shut it down. If it restarts again, just shut it down one more time. After this, it will stays quiet until you ask it to reopen.

from:  https://discussions.apple.com/message/10465608?messageID=10465608#10465608?messageID=10465608

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iamgamAuthor Commented:
Thanks - that's looking promising. Will monitor for a few days & then be back to award points if it's a keeper...
iamgamAuthor Commented:
Thanks for this. Apologies for the delay in awarding points.
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