Sounds as if your problem is the result of overusing your brakes and the accumulated brake dust settled in the sensor area. This is probably in your front wheels as they put out the most dust. Remove your wheels, calipers and brake discs. Behind this is the sensor and the ribbed hub that it 'reads' or creates the signal. Over time this ribbed area collects dust and signal output problems. This of course is if it passes the test that M.B.Doc mentioned previously.

Good luck,
Tobias MB
300CE, 400E, C280Sport, 190/5.0