iMac phantom power! It comes on by itself.

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and toggle the power up automatically off and on to see if the problem persists. Reboot normally and test to see if it takes.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

Well you have done some good troubleshooting, please do the following and we can look for obvious software issues.

It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click How to use the Add Text Feature When Post… - Apple Community It is helpful to us if you download the report to your Desktop.

It downloads to your downloads folder. If you have not relocated the downloads folder which is located in your user folder. Double Click your user folder and you should see a Download folder, Double Click the Downloads folder and you should see the installer for EtreCheck, Double Click that and it should install.

If you cannot get it to download that means something is VERY wrong with your system and you will either need to take it in for service or you can start by calling AppleCare and maybe they can start diagnosing over the phone. If you have never called AppleCare, please locate the phone number for your country by clicking AppleCare Contact Info

BTW, do you have any of the following third party apps installed on your Mac?

·      Antivirus Apps

·      Cleaning Apps

·      Security Apps

·      VPN Apps

·      Maintenance Apps

Then boot into Safe Mode, make sure the boot automatically is not turned on and then reboot normally. The booting into Safe Mode does the checking and fixing mentioned in my previous post.

Why are you shutting down the mains each night. Keep the Mac plugged in but shut down. That's what it's designed for.

What exactly do you mean EtreCheck will not download? Be specific we cannot look over. your shoulders and rely on your description to get a clear picture of what you are experiencing.