Dealing with a very similar issue, here, right now. My guy has had the chronically loose stools since he came to the barn (had been a neglected pasture puff for a year before that, so lord only knows what he got into there). We tried a supplement with more probiotics, and it made no difference. I got the vet involved, and he suggested, as a first pass, Probios. We did that for a month, and it made no difference. Then I had the vet back out for shots and stuff this spring, and he did some acupuncture (which definitely got my boy's batteries revved up) and 5 days worth of metronidazole. One, or the other, or both worked a Wonder - his poop dried up and looked like a normal horse's, and I had no nasty stuff running down his hocks, and he stopped squirting when he farted. Unfortunately, it all started to creep back a week or two after we finished with the metronidazole.
We didn't know whether it was the acupuncture or the metronidazole that worked, so we tried acupuncture again (since it's easiest on the body), and that didn't have an appreciable effect on the poop (although, it definitely improved Huey's energy and focus). So now we're going to try another longer round of the metronidazole.
It's entirely possible that he picked up giardia during his "cinderella" year. I also noticed that - even though Huey has a good BCS and doesn't appear to be a "hard keeper" - during the period before where the treatment worked and his manure firmed up and all, he looked like he was gaining a little weight. I was surprised at what a difference it made having this problem cleared up even for a little while.