Did you guys fiddle with /etc/ttys or the way to start X?
This looks like the know problem of X and the process on console fighting
for input because they are sharing the same virtual terminal device.
Normally there's no getty(8) process attached to ttyC4 so that X can attach
to it (and xdm isi configured to use vt05 - which corresponds to ttyC4
since VT numbers in X start at 01). If you activate a getty(8) process on
this VT too, X will not find a free one and attach to ttyC0 anyways causing
problems if you have processes trying to get input from ttyC0.
