Okay, dogs can definitely have changes in digestion or develop food allergies as they get older, but they can also develop serious problems like intestinal cancers that cause similar symptoms. However, if this has been going on for 2 years and you have seen improvement with the treatments then serious problems, like intestinal cancers, are less likely.
The diet, pepcid, and probiotics, like fortiflora are my first line of recommendations. If they have diarrhea associated with the bouts and the fortiflora doesn't resolve it then I sometimes add an antibiotic call tylan powder. The only other thing I sometimes try is Vitamin B12. This has to be given by injection. Initially it is every few days, then it is every 2-4 weeks after that. If you are not sure if vitamin B12 will help, you can run blood work to check the vitamin B levels. If they are low then it will definitely help.
If that doesn't help, then sometimes another diet will help. If they have a food allergy they will frequently do well on a diet change, but after some time... 6months or more... on that diet they can develop an intolerance to that diet as well. I know you just started the fortiflora, give that time to work. If it doesn't work consider measuring vitamin B levels and trying another idea change. I like Z/D (it is a hypoallergenic diet) as my second choice. I/D is my first choice.
If nothing helps, then it is time to do some more tests to make sure there isn't something more serious going on.
I hope that helps, but if you have more questions please let me know!