iMac continual restarts and frozen screen and Apple Store advice

My wife’s iMac is badly misbehaving and her tech support, that would be me, is not making much headway nor did The Apple Store.

2018 iMac 8GB/256GB SSD/A1418

The Mac was running High Sierra.
She would often find it with a frozen screen and on a forced restart it would report there was a problem and it restarted.
I decided it might be the OS so I upgraded to Big Sur.
Very similar issues followed.
Reported this to Apple Care and they had me take it in to local Apple Store.
They wiped it clean, reinstalled Big Sur and kept for several days.
At that point they said it was running fine. All diagnostics showed no hardware issues.
Using Migration Assistant I transferred her files from the external Carbon Copy Cloner copy.
Same problems returned.
I’m now told by Apple to start totally from scratch. They concluded the user was corrupted.
They didn’t want to see the kernel panic report after each restart. I don’t know how to interpret it. Would it give any clues?
Before I invest multiple hours, I’m curious if experts here feel that I’m getting the correct advice and I should proceed to wipe, install Big Sur, set up her user account, and then install applications one by one.
I’d then use her Dropbox and iCloud syncs for most of her files plus drag copy others.
Thanks for in advance for your advice.

Your own diagnostic process – it works ok without a certain user; it fail migrating in a certain user – indicates that Apple’s advice is correct. However, you did not indicate migration of any applications, so some diagnostic data is missing. This is a situation where a secondary administrative account is useful. The problematic user account can be deleted without deleting any migrated applications. ---- Most likely, the /Library contains corrupt files. Rather than using Migration Assistant to import her account, use CCC or something functionally similar to copy everything except the /Library folder. Any account-specific preferences for networking and applications must be manually re-entered. Any iCloud or Google shared data should automatically be available. Any IMAP mail will automatically be restored from the providers servers. As noted elsewhere, restoration of Mail > On My Mac folders is best done within Mail.

Editorial – The best preparation for situations like these is to compulsively save software keys, logins, and passwords as each is entered. I happen to use 1Password for this purpose, but there are other apps that can serve.

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Thank you very much.

I’ll begin what I fear will be a very long project for me. :wink: LOL Off to wipe and install Big Sur.

You say: As noted elsewhere, restoration of Mail > On My Mac folders is best done within Mail.
Would you please explain, I’ve now upgraded her travel Mac for her use while I fuss with the iMac and the Apple Mail folders are not what I expected. She uses two Gmail accounts but one forwards to the ‘main’ one. I’m a couple of years post using Apple Mail (now using Mailplane), so I’m a poor tech support for her re: Apple Mail app. Thanks for any suggestions.

“As noted elsewhere, restoration of Mail > On My Mac folders is best done within Mail”

But how, if there’s no usable Time Machine backup? I have the On My Mac mailboxes, but Mail doesn’t recognise them and so refuses to import them.

Have your tried importing them through your mail account provider’s web interface? I use Fastmail. I imported successfully all of my “on my Mac” emails and email folders that way. I don’t remember exactly how, unfortunately. I think I simply dragged them into my Fastmail account online. Now they not only are “on my Mac” but also available on all of my iDevices. This may not be what you want, but at least I have access to them. You might then be able to put them back into strictly “on my Mac”.

I have had multiple random restarts for months now. Usually when the CPU is under heavy use, although some seem random. Apple has been unable to find the problem or a fix. I had thought it a hardware problem but now think it a OS problem that they are hiding from.

Your experience with this issue does not give me a good feeling about tackling Apple’s recommended fix of wipe clean and do a clean install of Big Sur and install all apps from scratch.