Probiotics are a good idea - they sure can't hurt. Try to get a wide spectrum one. Here's an excellent site on all sorts of natural cures for things like IBD etc - it's geared towards dogs but you may get some ideas for your cat:
Has she had any meds or shots lately that could be contributing?
Oh...and you may want to consider feeding about 25% canned cat food as well...it's better for them, especially if you have males. I believe it prevents formation of kidney stones...or something...?