My iMac only works in safe mode, in normal mode the screen layout is really weird. Please help! :)

You have a sensor error. You can make an appointment at the Apple Retail Store - Genius Bar and take it in for service. You can also look for an Apple Store - Find a Store - Apple.

Install El Capitan from Scratch

Restart the computer. Immediately after the chime hold down the Command and R keys until the Apple logo appears. When the Utility Menu appears:

1. Select Disk Utility from the Utility Menu and click on Continue button.
2. When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the side list.
3. Click on the Erase icon in Disk Utility's main window. A panel will drop down.
4. Set the Format type to Mac OS Extended (Journaled.)
5. Click on the Apply button, then wait for the Done button to activate and click on it.
6. Quit Disk Utility and return to the Utility Menu.
7. Select Reinstall OS X and click on the Continue button.

As you reported: only 1 failure message: 4sns/1/40000000: TH00-9.000. "SNS" means a sensor error. There are numerous sensors in the computer. I believe TH100 refers to a HDD thermal error.

I doubt that is the reason. I would get it checked out since you have stated you have a problem.

In addition to the Hard Drive, what else have you upgraded? Do you have any third party RAM installed? Try isolating the RAM to see if you have a bad stick. The other thing this could be is a faulty GPU. When you start the computer in Safe Mode you are not loading all the graphics drivers. Seeing as though you have done a clean install of the OS a corrupted driver is unlikely but it may be that a component on the graphics card that is only affected when running all the drivers has failed.