Check your iMac's firmware ... I have rebooted and made sure firmware was installed before boot process sequence. Had a similar issue happen also but once the firmware was installed the problem had stopped. Do not just think that resetting SMC is a cure all solution, although it is a good starting point. I would double check your issue by rebooting your iMac in safe (boot) mode .. then you will know for sure. Instructions on safe boot mode can be found via the net same with install of new firmware. If problems persist .. must be escalated to a problem w/ motherboard and/or replacement.. Call AppleCare if your are still under warranty. Best of Luck