There are some bug reports at Ubuntu about the latest microcode update...
... and Ubuntu has now had to roll it back to keep systems bootable.
Is there going to be a fix for these reboot loops from AMD?
For lack of a response, I'll provide an update myself.
New microcodes were posted to linux-firmware.git this week. Which is nice.
Unfortunately, no change log is available, so it is unclear whether users should treat it as having security or crucial stability impact (making it an urgent update), or how much testing went into it.
Please do consider providing at least a minimal change log for future releases. Thanks.
Have you tried a different distribution to see if the problem is uniquely with Ubuntu? I have tested several distributions is virtual machines mostly to see how they differ.
You may need to try one of the more recent non LTS versions and see if that is successful.
Check for a BIOS update as well, might be worth a shot.