In our case downgrade the microcode don't work, instead of that we noticed that our kernel was outdated: uname -r 4.15.0-51-generic so we upgrade the kernel to the recent version with:
sudo apt-get install --install-recommends linux-generic-hwe-18.04 xserver-xorg-hwe-18.04
with that the kernel was upgraded to: 4.18.0-21-generic and ubuntu start correctly
In our case downgrade the microcode don't work, instead of that we noticed that our kernel was outdated:
uname -r
4.15.0-51-generic
so we upgrade the kernel to the recent version with:
sudo apt-get install --install- recommends linux-generic- hwe-18. 04 xserver- xorg-hwe- 18.04
with that the kernel was upgraded to:
4.18.0-21-generic
and ubuntu start correctly