Were the brakes pulsing when the pedal was pressed or just driving down the road not on the brakes?

If pulsing when pressed, I think a wheel speed sensor is having an issue. Sometimes metal filings, rust gets on the sensor / tone wheel making the computer think the wheel speed is lower than normal triggering the ABS.

One way to zero into this is to add a set of lights to the ABS coils and see what one is actuating or even better use a 4 channel oscilloscope and watch the wheel speed sensors.

It would be tempting to unplug any BAS sensors that are not related to ABS function and see if the problem continues. I just got a 99 C230 parts car that has 2 sensors on the brake booster ( diaphragm position and vacuum reading I think. ) and a small computer under the master cylinder.