This has been happening to me also with the Bloomberg video (56k and/or 28.8k versions) both via the QT4 plugin in NS 4.5 and in QTPlayer.
The movies will play for several minutes, then stop and no data will be coming through the modem. If I hit the pause button, I get the error 454, but then I can start the movies up again, but within a minute or two, they will stall again. Mysterious.