I don't think it is the cable, since:
1) I used the original cable that came with my Iyama A902MT monitor,
which is brand new
2) the problem only occurs in X, not in textmode or in Windows (sorry
for that)
Those don't necessarily rule out signal degradation.
1) Iiyama may have given you a lousy cable;
2) Windows may not be cranking the signal as hard, and textmode almost
certainly isn't.
Ghosting is less of a problem at lower pixel clocks. I don't know
enough about field effects and signal stuff to know exactly why, but
this has been my experience.