You can test it by hanging an equivalent weight onto the ceiling fitting - risky to fit the new lampshade but that's what I would do...
Here's what happens...
http://www.youtube.com/watch?v=ZLy5sP4gl1g&feature=related
Alternatively you need to fit a wooden brace between the joists - either from the floor above (easier) or from the room itself - much more difficult because it will very probably need ceiling repairs afterwards.
You need a strong base for the fixing screws to bite into.
Take care...Good luck...
Peccavi
February 2009