Squeaky brakes

I own a 2000 Subaru Legacy Outback. Rear brakes were squeaking badly when I let off the brake pedal. Seemed to be worse the farther I drove (as they warmed up). Replaced brake pads and squeak went away for app. 1 wk. and then came back. Garage told me it sounded like I had warped roter. I replaced the roters and put the same new pads back on. Noise disappeared for about 2 wks. Checked calipers for proper movement. Seemed to be working OK. Anybody have any ideas?