Webplayer streaming just stops silently

Normally our streaming webplayer build works fine. However, far too frequently the download just stops part of the way through. There are no errors listed in the webplayer log, there is no consistent point in the % progress, it just stops. The game is not crashing, the internet connection is fine, and this has happened on IE, Chrome, Safari and Firefox, and on PC and Mac.

Sometimes the stall will happen in our little loading scene, so it is very visible (very simple coroutine loop checking CanStreamedLevelBeLoaded then yielding for a frame).

Other times it must be happening in the background while you're playing a level that was successfully loaded early in the stream. When you try to go to a new scene, CanStreamedLevelBeLoaded must be returning true, because in those cases the error is printed that the level is not available (and we do not get that far unless CanStreamedLevelBeLoaded return true).

Often if we close the browser tab and load it again, it will be fine. For players that is unacceptable; to them it just looks like the game crashed. We have no way of detecting, that I know of. What could be causing the streaming to stop, and how can we track it down?