Jumbled screen on FreeNAS installation on MacPro3,1

I´m trying to install FreeNas on a MacPro3,1, using two pens: one for installation and the other for running the system.

After creating installation pen with image (as explained on video using Balena) the system - an old but solid MacPro - boots up until first menu with perfectly correct graphics.

Then, after first menu, the screen becomes "out of sync"!

It looks like if the lines that compose the image are broken with wrong length. After that its impossible to choose disks, and so on, because the menus became unreadable.

After searching the internet, i found suggestions like: entering "mode 0" or setting kern.vty=vc, but nothing worked.

Additionaly: the command "gop" returns "no Graphics Output Protocol present" (maybe not exacly like this. I´m telling by memory);

and the command "uga" says that could not confirm the stride value (once again, maybe not exacly like this. I´m not near the machine right now, i can´t tell exacly whats the message)

I would appreciate any help, since now i´m stucked without ideas on how to pass the problem.

Thanks.