Dog has permanent runny stools. Advice? Experience?

I've tried food changes, probiotics, and he has been on several courses of antibiotics for giardia and parasites--after which he improves and then worsens again. He is one year old and his sister from the same litter has had some of the same symptoms but not nearly as bad. This is not just a switching-foods-gradually scenario: it is a persistent condition I want to get to the bottom of. Thank you for your advice! I hope to hear from those who perhaps have had similar, prolonged cases of this kind of condition. Andrew