I have a quad core iMac. I recently did a restore from Time Machine, and afterwards, would leave the machine to rebuild a large thumbnail database. However, it has been hanging after a while, with the system taking up a constant 25% of CPU usage, when NO processes take up any observable amount in activity monitor. Even when I'm forced to Force Quit this job, the system remains at 25% usage (by system), with 75% idle, and no process accounting for this usage until I restart. Can a core go 'bad'? I've tried reinstalling Mac OS X AND reinstalling my app, but this still happens, where it didn't used to happen.