Installing on iMac 24" (2008)

Hi guys,

i'm trying to install FreeBSD on my old iMac 24 (2008) dual boot with OSX El Capitan. I successfully installed rEFInd and i can boot with usb key img of 11.0-RELEASE but after boot manager, screen goes flickering (like in the photo) and i cannot procede in the installation process. Same result with 10.3-Release. Any idea ? Thanks for help.
Maybe can help passing some option to boot manager to use an old safe compatible video driver ? :-? I don't need any GUI or desktop manager but only a text console. I will manage it by remote ssh. Any help ?