I have similar issue. HBO won't answer my emails so here I am. I have a Roku 1 also have 100mbps DL/8mbps UL. The only channel I have issues with is hbogo, every show the sound is annoyingly choppy for about the first 5 minutes, I can skip back a frame and sometimes that fixes it though not usually. I have it connected via LAN or Wi-Fi and both react the same. I'm assuming this is either an issue with HBO servers or the Roku1 for some reason. But as I said, only having issues with this channel, every other channel loads HD in 1-3 seconds. Any help here?
If everything else is working as it should, it's very likely a slow link somewhere on the internet between your Roku and HBOGo's servers. Do you have the same problem all the time (prime-time and off-peek)?
Unfortunately, there's probably not much you can do. Some people have had luck changing their DNS settings (and getting a new path to the servers at HBO), but that could have adverse effects on the rest of your channels, and might not solve the problem at all if the problem is "closer" to HBO than it is to you.