imac keeps going through root shutdown -h now @ login

Not much of a Mac expert, but i have a 27' iMac , dual-quad core with 8gb ram. It boots into safe mode okay but at startup in the console viewer I get sudo root /PWD: shutdown -h now and then on the next line a random service. first it was location.d which i found and disabled , and then rebooted and then it was usb.d and another reboot it was something else. I ran a virus scan, and applejack and nothing worked. When I boot in safe mode I can login, I removed all startup items from /Library area and the only login item i had running was an iTunes helper which i removed too for good measure.

Basically , I can boot it, the system flips to a blue screen for a min, then back to grey and loads up a little more, then i get the login, if i wait about 1 minute the system will just turn off. , so doesn't seem to be a user login issue.

Any ideas appreciated. i was debating just renaming the shutdown command as a test to see what happens. there is no time machine , and the mac doesn't have anything special, it has office 2011 , parallels 6 , logmein , and cord (a remote desktop app). i removed log me in and parallels as a test already.

Sounds like a hardware problem if it's shutting down both in Safe mode and from a normal startup. Run the Apple Hardware Test if the machine will stay on long enough.

Possible problems which can cause a shutdown: Power supply, memory, overheating.

Sorry if I wasnt clear, booting in safe mode, the machine has been on for 2 days without incident. None of the computers here have the Diag , and I havent been able to run it. we have 30 iMacs and a Mac Pro all Lion 10.7.3 . After testing diag mode on a few of them , only the macbooks go into diag mode. The whole diag mode is a whole different issue, where as we have a hardware problem on the server and cant get apple support. My client is about ready to throw their whole server out and go back to Windows (this is a whole different topic for another day heh)

Thank you for responding. I have also looked at several different ways to run the diag test and never could get it working.

On this iMac I did the following

Remove startup items on all accounts
Remove startup items from the /Library (called system startup items?)
Ran Disk Utility (First Aid and Permissions repair)
Ran applejack from single user mode (both quick and long auto fix)
Installed iAntivirus and ran a full/deep scan.

All Im trying to figure out is if the machine hardware is failing or software (which i believe) . if its software than what is it. What is so horrible that the computer is creating its own shutdown command via sudo root, yet wont output the issue into the console