SP 1.6 freezing problem

[jace2006]

I bought a SP 1.6 G5 on ebay last month and have been experiencing intermittent system freezes which require a hard reset. No SPOD, no KP message, and nothing in the system log. Just a frozen machine. I've done just about all the troubleshooting I can think of--RAM swapping, disconnecting all peripherals, reinstalling OS (Leopard), installing Tiger, installing OS on new drive--to no avail. I've read a lot of posts here and in other forums from a few years ago detailing similar problems, and I'm hoping someone found a solution that didn't involve a logic board replacement. Any insight would be much appreciated. TIA