I completely refurbished my 1996 850's front brakes 2,000 miles ago. I replaced hard brake lines, added professionally rebuilt calipers, new stainless steel flex lines, new Brembo rotors, new pads and flushed the fluid with my pressure bleeder. I felt warped rotor like shaking of the steering wheel when braking, and took the car to my mechanic. He looked over the entire system and confirmed that the rotors were badly warped. I had really babied the brakes while they bedded in, and made gentle braking a habit for the short life of these rotors. I never hit the brakes hard, and mostly tried to coast to stops. Calipers are not seizing, pad wear is minimal, pedal pressure is nice and firm, no pulling to either side.
My mechanic says rotors aren't as well built as they once were, and that warping right out of the box can happen. I've already ordered genuine Volvo replacement rotors and pads. Is it just that the Brembo rotors were of poor quality and the genuine Volvo discs are the answer?

 

I've never had issues with Brembos or Zimmermanns - those are both OEM brands so its not that they aren't up to Volvo standards. Did you check if the rear brakes are in good shape? Its possible you got bad out of the box... were the calipers properly greased and would the wheels spin freely when you finished the install? seems kind of odd as rotors normally warp from too much heat so if you replace, I'd make sure there's no drag and that the calipers can retract fully etc.