Quick look restarts my mac

Suggest the User start with Points 1 to 4 and then Test

Still having issues, move onto Steps 5 and 6 and Test Again

Still no resolve - move onto Step 7 and follow instructions therein 

1 - Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.

2 - Safe Mode will also eliminate Third Party Software, extensions and drivers from loading. It will only load the Minimum amount of Core Apple Processes to allow the the computer to function at a reduced Level of Performance 

3 - Does the issue present in this mode ?

4 - Sometimes a Safe Boot followed by a Normal Boot will just put things right.

5 - If not - there could be something in the main User Account playing up. To further isolate this - Set up users, guests, and groups on Mac. Then log out of the Main User account and log into the dummy account and test again if the issue persists.

6 - If the issue is present in the dummy account - then, this appears to be a System Wide issue on the computer.

7 - Download the Application Etrecheck directly from the Developer.

This is a Diagnostic Tool that makes no changes to the computer.

It makes a coherent and readable inventory of both the Hardware and Software used on the computer 

The application is free or paid from added features. 

The Report will Not Reveal Any Personal Information. 

Post back the Full Report - copy and paste - >>>> using the Additional Text Icon ( 3rd Icon to last ) <<<<

Part 1 of 2

Any Third Party Applications that will interfere with the normal operation of the OS,  is an invitation for disaster. 

Certain Applications maybe available on the Apple Apps Store - this only means the Developer is prepared to pay Apple a portion on each sale. What the Application may do to the computer is up to the User to check this out before purchase

Any of the below should be removed as per Developers Instructions 

1 - Disk Cleaner 

This will include CleanMyMac , aka “ BrickMyMac “

MacKeeper,

Dr Cleaner Pro Plus

Open Cleaner one Pro

Macmechanic

Clean my Mac x.

2  - Third Party Security Software 

This will include BitDefender , 

This will included Norton Antivirus 

 Sophos Av Software

Intego AntiVirus

 McAfee, 

Avast AntiVirus

Ad Guard,

 Webroot ,

 ESET ,

 Avira ,

AVG AntiVirus  

 avira antivirus 

Trustee  

AntiVirus for Mac

F-Secure

Securemac

Cylance

Kaspersky Internet Security

Total Virus Software 

VirusOne

Remove Virus One 

Read some of the posting and arrive at your own conclusions.

The The Built in Security  is all that is required.

There have been some bugs reported with the QuickLookService process and associated processes. One thing you can try is to go to Activity Monitor and list all processes alphabetically. Kill every one that starts with QuickLookxxxxx . Then see if the improper behavior is gone.

Others have reported that this command entered in Terminal fixes problems with the Quicklook background processes. This command also clears certain caches, which can be part of the problem. Be cautious when using Terminal. Be sure you have backups, and be careful in copying and pasting Terminal commands:

/usr/bin/killall -KILL QuickLookUIService && sudo qlmanage -r

If you have any anti-virus, security software, "cleaning" utilities, or similar third-party (non-Apple) tools, I suggest that you first uninstall all such software and tools and restart your Mac before doing anything else. I would also boot into Safe Mode, then restart normally, to see if that resolved it.

If you do try manually killing the QuickLook services and background processes, the problem might or might not return after a reboot. If it returns, then it is most likely due to something that is installed conflicting with the MacOS.

If all else fails give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

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.

There is almost certainly some hardware issue with your mac.

The most likely suspect is the Fusion Drive. If you can install an OS in an external SSD and run from that without crashes, it would confirm an issue with the Fusion Drive. As you know, this is a combination of a tiny 28GB SSD and a 1TB hard drive.

I know you already had your mac tested and nothing was found, but these things can happen - something failing intermittently is very hard to properly diagnose if (as per Murphy's law) it works perfectly when you send it in to the shop...

Do as already recommended by steve626 and Okd Toad, and restart in Safe Mode.

If the problem returns after restarting in Safe Mode and then restarting normally, then, as steve626 said some third party software is involved. In that case Please run Etrecheck and post its full report here.

Use the “additional text” button and paste the report into the text box.

If you have the time and are willing to Test if the Fusion Drive is the issue

Link below to use an External SSD Drive to Startup the Computer

It will involve installing Sonoma to the External Drive and running the computer from that drive

It could be insightful as well as possible determining if the Fusion Drive is the culprit

Use an external SSD as your startup disk with your iMac

Only in the interest of Continuity

chombeaux’s Subscriptions - Apple Community