Hi folks, I've installed 10.1 on a 5000-260 (zoom!) with a PMAGB-B.
The framebuffer works great in ROM console mode and with wscons, but
when X starts up, it seems to change to a sync rate that the display (a
DEC fixed-frequency CRT) cannot sync to.
I don't have a specific solution but I can suggest a work-round. I ran into similar symptoms a while back when trying to start X on 9.3 and 10.0_BETA on my 5000/133 with PMAGB-B. Thanks to some suggestions on this list, what worked for me was to install the full X11R6 sets from 7.2, those worked with the generic 10.0_BETA kernel and I was able to start an X desktop.