Many have come to post in the Flat Panel iMac forum and haven't specified which version of Mac OS X they are using.
The Flat Panel iMac first models came with 10.1.2. Note below your Text of your topic field there is a pulldown menu you can select the version of X you are running. It is important since troubleshooting solutions to your problem vary depending on which version of Mac OS X you are using. Please be specific when asking for a troubleshooting response as to which version of Mac OS X you are running, if you have any additional third party software running at startup, if you have additional RAM and where you got it, and did you make sure the RAM was to spec.
Apple menu -> About This Mac will reveal the version of X you are running
Additional RAM can only be told by the hardware test CD that came with your iMac. The Apple System Profiler will not accurately specify your additional RAM.