Is there a reason why you suspect you may have parasites? Typically, they are not easy for people to catch in the US or England because we have very good hygiene. Only people who have traveled to 2nd or 3rd world countries, people who work with animals a lot, and people who have drunk from an unfiltered source of water (such as a river or stream), have to worry about parasites; most people in the West never get them their entire lives.
That fiber, calcium and probiotics seem to be helping you would indicate that your problem may be a bacteria imbalance in your system, not a parasite (I've never heard of parasites being improved by anything except very strong antibioitcs and drugs made to specifically kill them). I think you should keep up your regimen of fiber, probiotics and calcium and see if you don't continue to improve. If you hit a plateau and aren't getting any better but are still less than normal, then you can look at adding in more fiber if you're not already taking the maximum doseage; you can take up to three 600mg calcium pills a day; and you can add in probiotic pills in addition to the Activia. See if you just need a little more stuff to get you to close to normal.
Gatorade and other sports drinks (aka vitamin drinks, energy drinks) are not usuall a good thing for anyone to drink a lot of, considering that they have WAY too much sugar. And some of the drinks they advertise as sports drinks (not Gatorade, though) also have caffeine. Some IBSers have problems with excess sugar, although not all. Many more, though, have problems with caffeine. If you are just on the toliet all day, pooping water, then Gatordae might not be a bad idea; but try cutting it in half with water so you get less sugar per dose; you should still be getting enough electrolytes if you drink it regularly. If, however, your diarrhea doesn't have you on the toliet just continuously, then you probably don't need to worry about the Gatorade, provided you do drink plenty of regular water and you are eating. Also, are you taking imodium or pepto bismol or something like that to stop your diarrhea? Do those things not work? Generally it's a good idea to try and stop diarrhea with medicine rather than stay on the toliet all day and drink Gatorade. A serious purging of the bowels is not good for you, even if you do drink Gatorade, because you lose more vitamins and minerals than Gatorade alone can replace, not to mention making you feel horrible. If you stop D before you have too many trips to the toliet, then the Gatorade isn't necessary. Also, there is (at least in the US) a product made for babies and children with D called Pedialyte. If you feel that you need vitamins (you can't get your D to stop or you can't manage to eat), then look at the Pedialyte and see if it doesn't have less sugar; I think it does (because no one wants a sick kid with sugar jitters).