hd100 recorder quicktime file too large

At the end of each clip, there is a 5 minute still image of the last frame. what’s up?

Are you sure it’s not your system freezing up from 100% CPU usage? Look under task manager (ctrl-alt-del, task manager, performance tab). If so, try using an alternate player like VLC Media Player or Media Player Classic Home Cinema.
Also read up about DXVA.
http://mpc-hc.sourceforge.net/DXVASupport.html
http://nunnally.ahmygoddess.net/watching-h264-videos-using-dxva/