>> Did these boards work in the past with the 32-bit sparc ?
>
> I can only comment on mach64 on 32 bit Sparc, it was working fine. The
> desktop was reasonably responsive.
Did you try the mach64 driver that was reintroduced to the Debian package archive?
Can you verify which package version of the driver you have installed?
And can you provide the full X.org.0.log?