Quicktime Player just stops playing

Maybe you have the same problem as I, see thread: http://discussions.apple.com/thread.jspa?threadID=2305212&tstart=0
What you describe is what would happen on my case as the download of the trailer is extremely slow in Quicktime player even though your network and other software is ok. So the playback always stops when it runs out of the cached file. And then it painfully slowly gets some more data and stops again... 😟

When I try to watch HD video clips, e.g. Apple HD movie trailers from the Apple site, Quicktime Player starts up ok and starts to play the video. But a few seconds in it just stops

This actually sounds normal. Basically, the QT Player attempts to cache sufficient data so that it can play the entire movie smoothly/continuously once it starts. However, if the data rate supported by your ISP falls after the movie begins, it may empty the cache and stop playback until the player has re-calculated the amount of data required and cached sufficient new data to the local computer at which time the player continues playback from where you left off (unless you try to force an early restart which may cause the playhead to jump back to the beginning of the file).

I have a 10Mb Broadband connection, so should be fast enough ?... I note that other HD broadcasts, e.g BBC iPlayer work fine (and don't use Quicktime).

It all depends on the data rate at which the movie was compressed and the actual data rate at which your ISP is delivering the content at any given time.

o see if this might be the problem, try switching from a 1080P trailer to a 720P version of the same file. If the smaller file plays fine, then the problem may be the ISP. Remember, heavy internet traffic can affect the ability to deliver content at a consistently high data rate.