[X4U] Strange behavior of iMac: shuts down and restarts whilst "sleeping."

[Richard Bienvenu]

That's about it; this started a couple of months ago and is pretty 
annoying--mainly because I have to restart applications I usually leave 
on, e.g., Safari and ThunderBird.
I'm running OS 10.5.8 and can't upgrade to Snow Leopard because this 
iMac is a single processor machine. Someone has suggested that this 
might be caused by an imminent hard drive failure, but the drive seems 
to be ok, at least according Disk First Aid.

Any ideas, anyone?

Richard Bienvenu
Boone County, MO