Losing connection to MCU

Need some guidance please.

Equipment:
RPi 4 4GB
BTT SKR 3 EZ
RATOS 2.1

Keep getting (while printing and just idle) the following message:
"Lost communication with MCU 'mcu'Once the underlying issue is corrected, use the "FIRMWARE_RESTART" command to reset the firmware, reload the config, and restart the host software. Printer is shutdown"

When I press "Firmware Restart", most of the time I will get:
"Failed automated reset of MCU 'mcu'Once the underlying issue is corrected, use the "FIRMWARE_RESTART" command to reset the firmware, reload the config, and restart the host software. Error configuring printer"

When Clicking "Firmware Restart", sometime I need to do it 2-3 times before restart. But it always comes back without a full power recycle. So never need to restart the MCU, although I have done this many times as well, just incase.

What I have replaced:
- Moved Rpi onto genuine Raspberry Power Supply
- Replaced USB cable
- Replaced RPi 4 with brand new RPi 4 (old RPi 4 was the covid model with 4 different power diodes that gave issues and so want to make sure it was not related)

Printer can stand idle for hours and all of a sudden it losses connection with MCU, but then it could also happen in matter of seconds after startup.

So where to next? Firmware, Software or Hardware?