It sounds like you need "cross bracing" or "diagonal bridging" between your joists (if you don't have it already) and if this doesn't fix it enough you may need to put in a girder beam and posts to effectively shorten the span on these joists.
bridging instructions: http://www2.hometime.com/projects/howto/framing/pc2frm02.htm
A girder is a beam that you put under the joists that is posted up. This is usually done at mid span to reduce the bounce in the floor.
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