Look in the STB EMU subforum. There are many issues with the STB's, including version problems and player problems.
The requirements are higher for the HD channels. In some cases, depending on routing, 5ish mbps download speeds won't cut it, especially if attempting wireless streaming. Even if other HD add-ons are working, the demands of these HD streams can be too great for lower download configurations. That may not be the answer in your case, but it is at least a consideration. I seldom see any buffering when others are reporting it. I have 20+ mbps at a device that is running wirelessly, however.