Need help to make a Radeon HD6970 to work on a G5

Hello everyone, I would like to thank Adrian Glaubitz in advance for his
many years of continuing to support the PowerPC platform on Debian Linux. I
have a PowerMac G5 Late 2005, been using it for a few months now and was
using the geforce 6600LE video card that came with it, I compiled some
projects like DevilutionX, SpaceCadet Pinball, Nblood, Sonic decompilation
games Mupen64plus etc and made them run with the geforce, but the
performance is bad. Few days ago I changed it to a Radeon HD6970, it's a
Cayman XT, I have the xserver-xorg-video-radeon and ati installed, but when
loading LightDM or XFCE4, the image crashes with artifacts, I can switch to
a tty console, in dmesg radeon says GPU lockup in ring 0 several times and
in the end there is an error in drm:v1_0_ib_test and
drm:radeon_ib_ring_tests, listing in lspci -k , it says kernel in use:
radeon and kernel modules: radeon, amdgpu. I'm using
linux-image-6.10.9-powerpc64 (4kb pagesize?) I can make the screen work
with the framebuffer but no hw acceleration then.
What can I do to try to resolve this?

Thanks in advance
Best regards, Link.