I just purchased a very nice 1993 240. It has ABS brakes and occassionally the brakes kind of "slam". It happens at slow speed when the vehicle is almost stopped. There is a popping type noise from the front end and I can feel the brake pedal drop a little. It doesn't happen all the time, more often when cold. Any ideas? Is it possible that a caliper piston is sticking and releases suddenly? The car has fairly new pads, but it is obvious the rotors were not replaced or resurfaced when the pads were installed. I want to replace pads and rotors and am wondering about calipers also. Any help would be greatly appreciated.

Thanks