Is this across the board or only certain apps? My Chromecast plays YouTube and Netflix video flawlessly but Vudu HDX, for example, annoyingly buffers every five minutes.
I noticed last night while watching chrome/netflix that if I hit the power button on the side of my phone, the buffering stopped almost instantly, but would have to do it every 10 mins or so. annoying to say the least but fixed it for now.
Proximity and quality of the router seems to be the biggest factor with the chromecasts. After that would be network load and isp issues. What is your bandwidth? Does if buffer on your other devices closer to your router?
Sense you mostly watch HBO go, I would troubleshoot other things before blaming your isp. If it were YouTube or Netflix, I would say it is throttling.