what do you mean 1200KBit/s ? that is not very much - are you meaning a low or a high number?
can you get hold of the files?
you have to turn off ARD or it will simply not work- you have 2 triple heads and ARD previewing 6x1024x768 screen - this just does not work well enough.
End of story - doesnt matter about whether your setup will allow it -
you want to play back video properly?- or you want to use ARD? - its a simple choice.
ARD kills things across the board -whether you are playing back movies or doing nothing.