One of the wires going from the sensor in the pad to the unit in the car has a short. All the sensor does is provide a ground to the sensor unit. As the pad wears and the plastic cover of the sensor in the pad is ground away by the rotor, you get a ground. that is why is starts out flickering initially.
One of the wires, from the sensor at the wheel, to the sensing unit is shorting out. Try disconnecting each wheels wire harness and see if you can get it to stop. If it does, that is the wire that is shorting.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car