I sounds very much like a RAM problem and the only way you can prove it is to remove one of the 512 sticks and just go with the one.
If the problem still happens, change out the sticks and try again.
Your initial install of Leopard should have been an "Upgrade" and not an "Archive and install". It probably was as the A&I option should not have been available.
Try the RAM and let us know.
P
In February 2005, I purchased an e-Mac, G4, PowerPC, 1.25 GHz, 1.25 GB RAM (changed to 2 sticks of 512) 80 GB HD, Superdrive running OS X-Panther (10.3), with i-Life 05.
I am one of those experienced Windows users who switched to Macs.
This month, I decided to change OS from Panther to Tiger but, Leopard had been just released about a week earlier so, I purchased OS-X-Leopard and did an "Archive and Install" of Leopard. I meet all the requirements and, after adding additional memory, I should not have had any problems. But, the Mac freezes and I have to force it to shut down.
I called Apple Care 4 times. The first two times, I ended up in India and got disconnected both times. The third time, I spoke with an Indian whose English was unintelligible but, he finally told me to press 4 keys and restart the Mac to do a "PRAM". It did not work. The fourth time, I spoke with an American who knew Macs. He had me do another "Archive and Install" of Leopard again. It seemed to be fine but, it wasn't long before the Mac froze again and it keeps doing this. I am using my Win XP/Home computer to ask for help on C/Net.
The Apple Store just sells Apple Products. They do not service them. Not even memory or installations. So, I took my Mac to Fry's Electronics where I buy all my Windows products. It was Fry's who sold me the memory sticks and, reluctantly, installed them. Fry's also sells Apple products.
I really need to do something quickly. Either successfully install Leopard or re-install Panther. I wish that I had purchased Tiger which now is a great up-to-date OS but.... I cannot find anyone who has Tiger for sale. If I cannot do this, I will have to re-install Panther and i-Photo 05.
P.S. I also purchased i-Life 08 and i-Work 08 to install on the e-Mac after Leopard was installed and working well.
P.P.S. I did download updates for Leopard (4 including Keychains, i-Tunes and Quick Time and 10.5.l).
If this was a Windows platform, I would not have any trouble changing OSes or Upgrading an OS. But, I am not experienced enough with Macs to fix this problem. I did find a store, MacMedia in nearby Scottsdale that not only sells but services Macs but, they are expensive and I would also like to be able to do this work myself.
Thank you in advance for any help anyone can give me.

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