I migrated my old G5 Tower apps/accounts/data to my new 24" 2.8GHz iMac in early November, and had periodic freezes (from 5 minutes to 3 days) that I was having trouble resolving. To narrow the field of issues, I built a clean partition with Leopard on the iMac and proceeded to install my old apps and data just to use as a control environment.
I found that this control environment NEVER froze, so I knew I wasn't having a hardware issue, but my migrated account continued to spontaneously freeze periodically. To see what was happening, I turned off the screen saver and energy saver, left the CPU always on, opened the Console log and Activity Monitor window (selected highest CPU usage column), left iTunes running, and waited.
The next time it froze, I looked at the Console log on screen and noted nothing usual was happening when it froze. Activity Monitor showed the iMac was idling at ~6%. Interestingly, I was able to connect ONCE to the iMac from another Mac and play an iTunes song, but multiple selections failed, and then the iMac became unresponsive over the network.
I read on a forum of another user who was having sporadic freezes and thought Hardware Monitor might be involved, but he couldn't confirm it. When I rebooted from my control startup partition, I noted that I hadn't yet installed Hardware Monitor on it, so I thought I'd try turning it off on my migrated startup volume.
That was nearly four days ago, and the iMac has remained fully operational and functional.
I initially tried just turning off the enhanced sensors extension, but that didn't help. My iMac ONLY remained stable with Hardware Monitor turned off.
Since it takes anywhere from 5 minutes to multiple days for the freeze to occur, it's difficult to diagnose, but I intend to reactivate all my usual stuff (except for Hardware Monitor) later today, install the pending Apple updates, reboot, and wait some more. If it freezes again, WITHOUT Hardware Monitor running, I'll be sure to update this post. But, it's never survived more than three days before, and it'll be four days this afternoon.
I should note I've been using Hardware Monitor for years, and it works just fine on all my PPC Macs (all also running Leopard). But it definitely appears to be having an issue with my new iMac.