many times when I download a movie there is a problem with the playback. For example, the movie will open and play, in "quicktime", the bar moves across the screen as the movie plays. However, if the movie is, for example, 1hr long, about 5min will play and it appears you are at the end of the movie because the marker is at the other end. But, if you click back about half way into the movie, it actually moves forward later in the movie. I've tried everything.
Any ideas would be appreciated.

I've occasionally had that problem but only in Apple's QuickTime Player. Try using a different player. I use VLC media player: it's freeware which supports many video and audio formats.

http://www.videolan.org/vlc/