Thank you for the prompt reply.
What is sound like you might be describing with the very intermittent GI signs could be a case of colitis, or inflammation of the large bowel. The signs can be VERY intermittent and would explain why she returns to normal and then the signs recur. The signs with this condition can vary in severity, from normal stools to mushy to very soft, all even within one day at times. Seeing mucus or blood is not unusual as well in some cases, but often there are only changes noted in the character of the stool (softer or discolored) There can be a sense of urgency to move their bowels and many of the pets will strain when having a BM, which to the owner can almost look like they are constipated. Many pets with colitis have GI discomfort and some will even vomit.
Colitis can have many causes including intestinal parasites, dietary indiscretion, food allergies or intolerances, metabolic disturbances from internal organ malfunctions, viruses, toxins, cancer or even stress. It is not accurate to assume that because none of the other dogs do not exhibit the signs, intestinal parasites may not be involved. Intestinal worms, particularly whipworms, are very commonly involved in leading to low grade inflammation in the bowel, with intermittent signs. They can be a challenge to confirm as the worms are not proficient egg layers and do not pass eggs daily, so if a fecal sample is checked on a day they are not passing eggs, it can be missed. It is for that reason, that when GI signs are present, it is very common to threat with a therapeutic deworming just so we don't miss it.
You are correct that in most cases of GI disease, bloodwork can be completely normal, but it does help to rule out other metabolic diseases that can have an effect on the GI tract, such as liver, gall bladder and pancreatic disease. Xrays may or may not be helpful, but again, rule out other possible non-GI diseases that can affect the GI tract. White stools can be caused by lack of bile pigment, so an ultrasound evaluation of the gall bladder and bile duct may be helpful to rule out possible obstruction. Being that she is a fairly young dog still, food allergy has to be on the list, and perhaps a food trial to rule that out may be a good idea.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.
My posted replies are for general education only and not meant as a diagnosis. Only after a thorough veterinary examination can a diagnosis for your pet be made and specific treatments be advised or medications be prescribed.