Please try the following:
Please let me know. Thank you.
Sorry, can you make sure all updates get installed?
Can you click on the Apple logo / menu, then About this Mac, then More Info, then Hardware, Graphics/Display, and let me know the graphics model there. Thank you.
Yes, I am; I support windows and mac and iPad, etc. I'm sorry, but I'm not sure about this monitor issue with El Capitan, so let me open the issue to other experts. Thank you.
What model display is it and what port on the iMac is it connected to?
1. Shut down the computer.
2. Unplug the computer's power cord.
3. Wait fifteen seconds.
4. Attach the computer's power cord.
5. Wait five seconds, then press the power button to turn on the computer.
6. Press and hold the Command-Option-P-R keys before the gray screen appears.
7. Hold the keys down until the computer restarts.
8. Release the keys.