Try just burning one DVD disk on one of the DVD burners in your array. If the speed is back to 8X then your throughput on your MoBo is just not fast enough to feed 4 mpeg data streams at the same time for the (4) 8X burners you have. That would be a "bus line bottlerneck" issue.
Another possible problem… and I say "possible" could be available power from your power supply. With all of those devices hooked up, your system could be starving for power thus slowing it down. I would think you would need at least 450 watts or more and even that would depend on what kind of CPU and video card you have not to mention other components installed.
RAM
