Buddy (6 months) had the same problem- inconsistent diarreah for about a month and a half. It started when I switched him (slowly) to Canidae. The Metronizodole worked but only kept the diarrhea at bay for a week or two at most. After a few weeks, he was also put on the Eukanuba low-res but then, after a week (was still on prescriptions), got sick again. At this point, we thought it could be a food allergy (the most common ones are to a type of protein, wheat). After a week on prescription allergy food (it is made of soy and looks and smells like Corn Pops!), I switched Buddy to California Natural Lamb and Rice Puppy with a tablepoon of pumpkin and he has been fine. I think the simpler formula of his current food keeps his tummy more settled. Also, it only has lamb protein and rice and a few vitamins (good for sensitive tummies!). I have slowly incorporated Bully Sticks back into his diet and have found a nice treat that is almost pure chicken (Pro-Treat Freeze Dried Chicken) and he has not had any problems
Good luck with Riley... I know how frustrating it is! Don't expect the vet to diagnose diarrhea- neither mine nor zack's (from what I read) were really diagnosed with anything except colitus. Keep Riley on the chicken and rice for a few more days and try the simpler food (lamb is supposed to be easier too on the stomach)- I think Natural Balance also makes a Duck and Sweet Potato Formula for allergy sufferers.
Let us know how it turns out!