Sounds like your experiencing the "drawbridge effect". Some how or other the rear pads set up a pulsating resonance with the fronts. It's especially noticable at slow speeds and moderate braking. There have been many posts on this and the "fix" seems to be the correct Volvo pads and shims, especially on the rear. I think the shims are different depending on whether the rear pistons have a cutout section on them. A while back one brickster even started a web page devoted to this problem in hopes of coming up with a good fix. it's that common.
Front rotors are good for at least two sets of pads and the rears maybe three. The general opinion is don't turn the rotors, just run'em tell they're worn out.