well you might want to try activating the QoS engine (Quality of Service) on the router...
in simple terms QoS is a way to give certain tasks or computers priority over others.
check this link out for help with the setup
How to configure QoS on D-Link DIR series router(DIR-625)? | D-Link Blog Home
ill admit im by no means a pro on this topic, no one else seemed to jump on this thread so id see if i could give it a shot, so in the very least i can give you a starting point.