What is their diet like? For the surge of mild colic is it possible there is something in environment causing them to have a bad reaction? I am in South Florida as well and all the rain we have been having maybe something has been introduced into their environment. Pesticide run off, a new weed growing, grass is very wet, or maybe just the weather being so wet.
As for the one with chronic diarrhea what have you tried already besides the Dia-gel. I may repeating what you already tried, has the vet checked for ulcers? Regardless I would feed beet pulp, with timothy/alfalfa cubes, no grains what so ever, if you use a commercial feed use one without corn, oats, or wheat. I would also be feeding pro-biotics, and a good mineral supplement.
If you have already tried that have you had your grass/soil checked for possible contaminants. Otherwise I have no clue.
Where in SF are you by the way?