There are A LOT of causes of chronic diarrhea. So many that I am sure I will miss a number of them in the following list, but here goes: malabsorptive diseases (anything causing an inability to absorb food, such as wheat gluten sensitivity or plasmacytic-lymphocytic enteritis), maldigestion syndromes such as pancreatic insufficiency, inflammatory bowel disease, intestinal parasites (protozoans, worms, coccidia), fungal infections (rarely), damage to vital organs such as the liver or kidney leading to secondary diarrhea problems, diabetes mellitus, hypoadrenocorticism, small intestinal bacterial overgrowth, cancer, ulcers, bacterial enteritis or colitis, cecal inversion and irritable bowel syndromes.
It is important to try to decide if this is a small intestinal problem or large intestinal problem. In small intestinal disease the diarrhea tends to be pretty large volume at one time but less frequent episodes of diarrhea. In large intestinal diarrhea the volume of diarrhea tends to be smaller but episodes are more frequent. Knowing which of these is likely to be the problem helps in deciding which tests to run.
In chronic diarrhea it can be very hard to find a diagnosis. The intestinal biopsies were are very good idea. It is too bad they were not more informative. It can take multiple fecal exams to find some parasites, particularly giardia and whipworms. There are pretty good blood tests now to aid in the diagnosis of pancreatic insufficiency (blood trypsin-like immunoreactivity, folate levels, it is always wise to do general lab panels to make sure other organ systems are functioning properly. It can be helpful to rule out food sensitivity/allergy using special diets (elimination diets). Culture of the stool will sometimes reveal a bacteria that is very likely to be pathogenic, such as Salmonella or Clostridium perfringens. Special cultures can be done for Campylobacter.
If sulfasalazine does help, that is a hint that the problem is likely to be in the large intestine. If biopsies did not include the colon it may be possible to obtain the necessary biopsy samples from the colon to make a diagnosis using an endoscope with a biopsy forceps.
In most cases, it is possible to find a cause for the diarrhea and to successfully treat it, if you are patient enough. You can tell from the long list of possible causes that it can be hard, though.