I had the same problem with my Phantom,
and according to the clock speed, there
were more or less stripes and flicker.
(at 66MHZ the screen was pretty ugly)
I think this has to do with the access
time of the memory. 100ns RAM seems to
be a bit too slow when overclocking.
Solution should be replacing the ST-ram
with some 60ns chips, and maybe put some
fastram in? I believe that should help.