Try worming with a longer course of panacur, giardia is a parasite that is often overlooked, can be picked up from water courses and "foxy" areas. Needs 50mg per kg bodyweight, daily for 3 consecutive days.
Some dog have dietary sensitivities, usually caused by gluten intolerance, so cut out all wheat/cereal products.
Some highly strung dogs just have "fast" guts, where things move thro a bit too quick, is the dog kenneled with others?? If nothing else works, might try changing the living arrangements but try the simple things first!! Remember common things occur commonly, so always start