Reading you question... I don't see any problem...
Whe I specify overflow-x:hidden; on an element, it adds a vertical scroll bar.
If it overflows in it's height (as you just said it does), then that's quite normal.
I have tried adding overflow-y:visible; and even just overflow:visible, to no effect.
Well... That's normal, as you're telling it to show a vertical scrollbar, wich there already is.
As kuloir said: X = horizontal; Y = vertical.