I was having the same issue & contacted HBO Go support. Here's the response I got & this seems to have worked for me -
Thank you for your email. We apologize for the inconvenience; we are aware of intermittent video playback issues on Roku. Our team is currently working with Roku to investigate the issue and hopes to have it resolved soon.
If you are experiencing video playback issues we suggest deactivating and reactivating the HBO GO application and confirming your connection speed.
Please follow the below:
1. Remove the HBO GO application from the device
2. Restart the device by disconnecting and reconnecting the power supply
3. Install available Roku software updates
4. Reactivate the HBO GO application
5. Test video playback using a hard wired connection
6. Confirm your connection speed, go to http://www.speakeasy.net/speedtest
7. Test video playback at multiple points of the day (morning, afternoon, evening)
If you are experiencing intermittent video playback issues a slow or inconsistent network connection could be the cause. HBO GO requires a very solid internet connection while streaming video. HBO GO will adjust video quality based on the strength of your network connection. We suggest contacting your internet service provider if you continue to experience video playback issues.
The HBO GO Team