Well, I thought it was cool - an older G5 iMac with newly installed Leopard ran fine for a week. Came in today and it endlessly reboots. gets to the desktop then 5 seconds later reboots, over and over. Launched into Single User, repaired perms, etc. Any ideas on other remedies to research? Could a PRAM battery cause this? Does the iMac have a battery? Sheesh.
Q

Can you boot from another HD that has an OS on it???

First impression is that something is corrupted in the startup manager. Was this new install of 10.5 an Archive and Install or Erase and Install? What programs or software updates have been installed since then?

Give us some more info on this system to help us help you

D: No, not an archive and Install but an upgrade. Right now it's running??? I ordered some PRAM batts in case. Only Apple updates - I just wanted to see if an older iMac played nice w Leopard but I'm less than impressed as you can imagine. I have external boot drives but they are all Tiger (primarily what we use here). I might try a Target Disc mode from a MacBook that is running Leopard and see what's what. Not sure if I have a Disc Warrior for Leopard tho' (my go to repair app).
Q

I would start with booting from an external, as Dave suggested.

Have you reset the PRAM? How about the resetting the System Management Unit (SMU)? Have you booted to Safe Mode and let the system repair itself?

I'm going to wait on the SMU until the replacement PRAM batt arrives. Safe mode - yes, no effect. But it has been up and stay up for over an hour now. What changed? Just my attitude apparently. Thanks.