When they shudder it is a really bad one. I took to my regular Volvo mechanic and he said it was the pads and changed them out and turned the rotors. The problem went away for a long time, but has returned. It seems to happen after the brakes get hot. If I brake hard they shudder. If I break easy, no problem. I checked the on board computer and it gave me a code for a bad/shorted ABS sensor and for heated brakes. I pulled the sensor on the left rear wheel and it was caked with a thick hard grease. Cleaned it checked the pads on both sides and hit them with emery cloth as the pads appeared to be glazed. Have only driven the car a few miles since, but no problem so far. Will know better this weekend as I have to drive some 50 miles through heavy traffic.