Some progression!
I followed the instructions in the section "problem: Need to fully remove -fglrx and reinstall -ati from scratch", in this link:
https://wiki.ubuntu.com/X/Troubleshooting/FglrxInteferesWithRadeonDriver#Problem:%20%20Need%20to%20fully%20remove%20-fglrx%20and%20reinstall%20-ati%20from%20scratch
The result is: color lines still appear but are automatically followed by the login screen.
Obviously this bug is related to previous installation of the proprietary fglrx driver. What remains is an ugly boot, but work perfect!
Some progression!
I followed the instructions in the section "problem: Need to fully remove -fglrx and reinstall -ati from scratch", in this link:
https:/ /wiki.ubuntu. com/X/Troublesh ooting/ FglrxInteferesW ithRadeonDriver #Problem: %20%20Need% 20to%20fully% 20remove% 20-fglrx% 20and%20reinsta ll%20-ati% 20from% 20scratch
The result is: color lines still appear but are automatically followed by the login screen.
Obviously this bug is related to previous installation of the proprietary fglrx driver. What remains is an ugly boot, but work perfect!