Random freeze of iMac screen after upgrading to Mojave

Looked at the Etrecheck Report and it is less than encouraging. Firstly, this is a Fusion Drive setup with a very small SSD 24 GB which, as per the report, is almost 100% Full. The Rotational Drive is a very slow 5400 RPM drive. Right there is the bottleneck. Not matter what is loaded on the drive, it is going to be slow. Add to this is all the other installed Application loaded. Running a VM Software requires a much more robust machine with plenty of RAM, more than what this machines has.

Would say - for better performance - purchase a blood, reliable external SSD large enough for your needs and install Mojave on that drive and run the computer from the External Drive all the time. Further, if going this route, do a selective installation of your Required Software. There is a lot of unneeded software installed showing down the computer.

Below is a copy and paste of things to consider Not reinstalling if running from the Internal Drive.

Time Machine backup drive is too small.

System modifications - There are a large number of system modifications running in the background.

32-bit Apps - This computer has 32-bits apps will not work on current versions of the operating system.

 disk0 - APPLE SSD AP0032H 24.00 GB (Solid State - TRIM: Yes) 

disk0s2 [APFS Fusion Drive] 23.69 GB

APPLE HDD HTS541010A9E662 1.00 TB (Mechanical - 5400 RPM) 

1.02 TB (Shared - 871.52 GB used, 160.86 GB available, 141.70 GB free)

File /etc/sysctl.conf exists but not expected

  /etc/hosts - Count: 21

Launchd: /Library/LaunchDaemons/com.seagate.TBDecorator.plist

    Executable: /Library/Application Support/Seagate/TBLoopDriveParams

aunchd: /Library/LaunchAgents/com.intego.backupassistant.agent.plist

    Executable: /Library/StartupItems/Intego Backup Assistant/BackupAssistantAgent.app/Contents/MacOS/BackupAssistantAgent --launchd

Applications/GlobalProtect.app

Seagate Storage Driver.kext - com.seagate.driver.PowSecDriverCore 

Delete these files & restart...

~/Library/LaunchAgents/cn.macx.MacAppBoxHelper.plist

~/Library/LaunchAgents/com.adobe.ARM.***.plist

/Library/LaunchAgents/.svn/text-base/com.qvod.agent.plist.svn-base

/Library/LaunchAgents/com.qvod.agent.plist

/Library/LaunchDaemons/com.cisco.anyconnect.ciscod.plist

~ tilde indicates hidden Local Library

Apple hid the Users' Library folders...

Method 1:

1 From the Finder, select the Go menu at top of the screen, and choose Go to Folder.

2 In the window that opens, enter~/Library/LaunchAgents, and click Go.

Suggest downloading the Application Etrecheck directly from a Trusted Developer and well Respected ASC Contributor. The application is free or paid from added features. Run the application with Full Disc Access ( Security & Privacy - Full Disc Access ). It will take a Snap Shot -  both the hardware and software. 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 )

We can have a look at the report for possible issues and may have possible suggestions to resolve the issues.

Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and may require a Reinstallation of the OS.

Between what I mentioned and what @BDAqua has suggested - it is apparent there is a lot of unnecessary software installed on the computer that is Interfering with the Normal Operation of the computer. Any one or several of the Applications, Extensions and or drivers installed could be to source your your issue.

It will require a great deal of work by the user to clean up the system to put things right.

I'd start with deleting the files I mentioned & restarting. :)

Agree with @BDAqua suggestion and let us see what that's works and report back - it time allows. Then, can determine if more drastic actions is in the offing.

:)