When you are slowing down, and let off on the brake (but not all the way) then press back down again, the pedal will to a very small shake for a second. It doesnt do it all the time, mabey a couple times a day if all. Its kind of like when the ABS kicks on when you are sliding on ice, but on a way small scale.

Any ideas ??

 

Possible, but I doubt it. Sounds more like your front rotors are out of round, and need to be turned.