I use tap water. And yes, only once a week. These guys don't need to be too moist. I don't have it in the same place as my vandas, as I hang most of those above the kitchen sink. The Phychopsis actually sits on a little table closer to the sliding door with a bunch of other orchids, so maybe it just likes company!
Maybe you could post a picture of yours? Are the roots in good shape? How old is it?