Help w/ strange happenings...please?

One of my Maybe-Maine Coon Cats has an on and off again issues with her stool. She would have the tendency to leave me a present in several areas of the house-(this includes the tub and bathroom sink !) but not the litter box. These would be very loose, but not watery. I would also notice her cleaning herself (around the anus area) she would start to growl and get pissed-off. This would go on for several weeks and stop for a while then begin again. Vet said that he could find nothing wrong after performing several tests. I'm kind of running out of ideas. (I'm thinking maybe hair balls ? or food ?) any suggestions will be helpful. Also- both cats seem to go an awful amount of poop-is this normal?

Thanks in advance

What do you feed them?
How many litter boxes do you have?
Where are the litter boxes placed?
How often do you clean the litter boxes?

(Sorry for the many questions, but all of these things can be significant.)

fstop61, if you haven't yet, after answering Antonia's questions, I would advise changing there food. Most "off the shelf" type of cat foods have all kinda of non essential and, sometimes harmful ingredients added to them. I feed my big boy only the best stuff I can find. Organic and, all natural, with no fillers, if possible. Best way to be aware of what you are feeding them is to read the label. If the list of ingredients has corn, rice, hydrogenated oils, of any kind, or, canola oil in them, I stay away from it.

This is just the beginning of being more aware of cat care. there are other things as well like, using biodegradable and, all natural litter. Different kind of food and water dishes, etc.

Hope something in there helps.