Carol, I'm sorry that your question wasn't answered in a timely manner and sorry to hear of this with Harry. The jelly you're seeing indicates colonic mucus. Excessive mucus indicates a colitis but not the cause of the colitis.
I would approach Harry's chronic diarrhea in the following manner although the order in which I've listed the various approaches can vary:
1) Presumptive worm with over the counter fenbendazole (Panacur) available in many pet/feed stores. Fenbendazole addresses all of the nematodes (roundworms, hookworms, whipworms) as well as the protozoan Giardia. This should be done even if a fecal ova and parasite exam didn't reveal parasites. That exam is too often falsely negative.
2) If worming weren't helpful, perform a gastrointestinal diagnostic panel of blood and urine tests. This should include a specCPL blood test (most sensitive for detecting the presence of pancreatitis), a TLI blood test (to check for maldigestion), and serum folate and vitamin B12 levels (to check for malabsorption of nutrients). Fecal culture looking for the gastrointestinal pathogens Clostridium difficile, Clostridium perfringens, Campylobacter, and Salmonella can be considered as well.
3) If this testing didn't reveal anything untoward, an abdominal ultrasound should be considered. Ultrasound is the most sensitive and readily available imaging modality for the gastrointestinal tract itself. My concerns would be inflammatory bowel disease, lymphangectasia, and neoplasia (cancer) such as lymphoma and solid tumors such as adenocarcinomas.
4) Perform a hypoallergenic food trial in an attempt to identify food intolerance. Food intolerance/allergy is addressed with prescription hypoallergenic diets. These special foods contain just one novel (rabbit, duck, e.g.) animal protein or proteins that have been chemically altered (hydrolyzed) to the point that Harry's immune system doesn't "see" anything to be allergic to. The over the counter hypoallergenic foods too often contain proteins not listed on the label - soy is a common one - and these proteins would confound our evaluation of the efficacy of the hypoallergenic diet. The prescription foods are available from his vet. There are many novel protein foods and a prototypical hydrolyzed protein food is Hill’s Prescription Diet z/d ultra (a hydrolyzed protein diet is my preference because it avoids the possibility of my patient being intolerant to even a novel protein). A positive response is usually seen within a few weeks if we’ve eliminated the offending food allergen. Food intolerance can arise at any age and even after our patient has been eating the same food for quite some time.
5. Scoping and biopsying of the gastrointestinal tract
Please respond with further questions or concerns if you wish.