I am experience a strange thing - the surface that I am moving with the "Move Face" command, suddenly dissapears in some parts of the drawing.

Take a look:

http://www.youtube.com/watch?v=oJJ_qk-tSjg&feature=youtu.be

What seems to be the problem?

Why can`t I move my surface to any point on the working plane?

Thank you for the help.