Mac 27 5K late 2014 unstable after SSD upgrade

Hi,

I've upgraded a late 2014 27' 5K iMac with an Crucial MX500 1To SSD.

(iMac 15,1 I5 3,5Ghz, 32Go RAM)

The Imac was originally running with an 1To Seagate HDD, fusion drived with a 128Go Apple SSD blade.

The installation was flawless.

But troubles came after the Imac was set up and running.

1st Case - FusionDrive running Big Sur

I create a new fusiondrive container with the freshly installed SSD, installed a Catalina copy and finally updating to Big Sur.

after a couple of hours using the mac, everything starts to slow down and the mac was almost unusable, "color wheel" and the mac take 5 minutes to open a simple folder ...

monitor activity show "kernal_task" using more than 250% CPU

Tested the two disk with driveDX => all OK results

2dn case - Catalina on Apple SSD blade

After issues trying the fusiondrive, decided to split it.

Installed a copy of Catalina on the Apple SSD blade and APFS formatted the Crucial MX500.

at the beginning, it seems OK, but randomly the mac goes black screen and wake up by moving mouse or keyboard (like it goes on sleep) (all energy save option have been disable)

NVRAM have been resteted

have trying to enable TRIM

Was confused, because the OS was running on the original SSD blade and have no clue where the issue are from.

any help would be highly appreciated.

Regards.

William