The FSR's I've worked on were prone to ghost-shifting to lower gears as the suspension compressed... might be a good excuse to try a SRAM ESP rear-derailleur/shifter combo, if you like twist-shift. It takes twice as much cable travel to shift ESP, making it theoretically less-prone to ghost-shifting.
However, I can't think why the cable would have more
slack when the suspension squats, instead of less
slack (as was typical of the ones I worked with)... unless perhaps the cable housing that joins the front and rear frames is too short, and it can't quite bridge the gap between the two housing stops until the suspension squats and the stops move toward eachother. Do you happen to have any photos of your individual bike that you could post?
This image is a bit bigger than I would want to put into the thread, but visit http://www.specialized.com/SBCBikePo...20FSRxc%20Disc
and look at the piece of rear derailleur housing that links the front and rear frames. Notice it arches up a touch. If yours is stretched taut when you're not on the bike, then you'd want to use a longer piece of housing.