Some people have noticed that there is a slightly higher pings with the CGN3... anywhere from 1ms to like 20some odd.. potentially higher than they should be.. but shouldnt be anything that effect streaming in the degree you are explaining.
There are two things i am possibly thinking here.
A ) how is the slingbox connected? And if its wired.. what is the speed port on it? 10/100 or gigabit?
B ) Your signal levels... could be a bit off. Are you able to post your signal levels from the modem?
I am on the hybrid 60 package, and dont have an issue connecting to my plex server remotetly and streaming.