loose stool

[hankers]

My dog has super loose stools but not every day, however atleast once or twice a week he will have it in his kennel. he is not a barker and he is a BIG boy he is a 90lb english cream retriever. we did take him in to the vet with stool sample it was negative for any parasites and what ever else they test it with... they gave us a pill for him to help stop the runs and gave is hills science diet i/d for a week... it worked we gave hime his food at teh end of the week and it started again.. the only thing that has changed since he was a puppy is we went from the puppy formula to the adult formula nothing else has change... he was on lamb and rice nutro natural large breed puppy and then simple went to the adult large breed form.

we are at our wits end as its a bit tiring that he has his accidents middle of the night.

help!