IMac freezes on first start up but after shutdown and second start is ok

Within the last 6 months our iMac started to freeze on every startup but after using the power button to shut down and after a second start up all is ok.
What could be causing this?
Carl

My first thought is the backup battery is dead or near dead. When you first boot up, it may be that because of the low battery your settings are corrupt and it freezes. On the second boot up, now that the system is all charged up and there is power to the PMU/PRAM (or whatever they call it now), that it has enough to properly reset or overcome bad PMU settings and restart properly.
If your iMac is over a year old and you have never changed that battery, now would be a good time to check that. One thing you can try is simply removing the battery completely. The PMU will reset every time the computer is powered down, but if the computer now starts reliably every time, then that was your problem.
Another possible troubleshooting cause...
Intel-based iMac, Intel-based Mac mini: How to reset the System Management Controller
http://support.apple.com/kb/HT1543
Patrick

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IMac freezes on first start up but after shutdown and second start is ok

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