iMac often shouts down sometimes without any message

Does you work station layout allow your iMac to "breathe?" A shutdown without an advisory can be due to the system overheating. Sensors first clock back the processor to reduce heat generation but, should that not work, it performs a full shutdown to save its brains.

iMacs take in cooling air through the bottom edge of the display:

so put nothing between those slots and the work surface.

Prior to 2012 the important hot air exhaust was a long slit between the stand slot and the top of the display. Starting in 2012, the exhaust was moved to a less-effective postion behind the stand:

All vents must be unobstructed to prevent damage-capable heat episodes. Overheating is worst in Summer in work areas lacking air conditioning.

This is a picture from a 2022 post here from a user who could not understand why his iMac was running so hot:

When I worked in law enforcement, we called that a "clue!" We advised the use to remove that stuff and he reported back that doing so solved his overheating issues.

Some useless third-party utilities can run up temps, as can some web pages. The EtreCheck report that our wise BDAqua has recommended will quickly show if that is an issue for you.

Hi there, only Apple can tell much anymore using Crash reports.

You can use Drive DX to possibly get a better view of Drive health…

https://binaryfruit.com/drivedx

Have you run Disk Utility's Disk First Aid on all connected drives?

In Disk Utility>View, select Show all Devices, highlight the top left entry.

Run Disk First Aid on all items in the left panel, from top down.

Are you running any VPN, Anti-Virus, or Cleaning apps?

We need to see what all is running, a report from this will not display any personal info...

Using EtreCheck - Apple Community

EtreCheck is a FREE simple little diagnostic tool to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac. It will not display any personal info. Give it Full Disk Access.

https://www.etrecheck.com/

Give etrecheck Full Disk Access before running.

Thanks for Old Toad’s etrecheck instructions…

Slow iMac 2017 - Apple Community

Earlier versions of etrecheck...

https://macdownload.informer.com/etrecheck/versions/

Ouch... Failing hard drive - This computer has a hard drive that appears to be failing.

Suggest getting an external drive or SSD asap, hopefully you have backups already‽

If you need suggestion just ask.

What does Drive DX report on all the drives?

It should list the Fusion drive twice...