This sounds like a classic problem of warped rotors. Often temperature has a large effect on rotor vibration. Slight warps may only be detectable once the rotors get warm. In your case they may be the rears rotors too. Do a simple run-out test on the rotors when they are hot.

My suggestion is when they wear out replace them with the stock Volvo rotors or go to ATE's slotted rotors if you must have the extra performance. Volvo brakes are and have always been overbuilt and are the most reliable combinations that I have seen (except for the late 700's). Cross drilled rotors are not as good as they seem. Often the holes allow cracks to occur when but to the extreme, and they are less reliable than some others. If these rotors are warped, do not machine them straight, just replce them and ensure that the wheel nut are tightened equally.

Andrew P