2004 Impala brakes pulsate

4 months ago, 2004 impala, (auto, front drive, 4 door, vortec V-6), started to vibrate when brakes were applied. Took it to Gdyear, they said brakes were fine. Took it later to Tir kng for A/C work, and they noticed the problem and suggested a brake job. I took it back to them, they did the brake job, and NO IMPROVEMENT. They have never encountered this problem. The brake job was a new pads, new hardware, resurface the rotors and test drive. They noticed no warp. (Great shop though, they charged me 0.00 for the brake job! What possible cause? No crash or known warn parts, I have regurally rotated and balanced the tires.
Any help?

Thanks ever so much. (Great forum by the way!)

Eddie