Power Assist Brake problems of the 1999 Hyundai Sonata

The transmission sensors have been replaced several times, I have also complained about transmission problems which dealer stated didnt exist until it went out completely. My biggest problem is with my brakes, they fail from time to time while driving, I once hit a parked car because I had no brake pressure when depressing brake pedal and pumping. Has also happened while in traffic almost causing an accident numerous times, yet dealer says there is nothung wrong with my brakes, I have brought this up everytime I bring my car in for all the other repairs. My airbag wiring is faulty and has been replaced 3 times which I feel is a serious safety issue. My engine stalls occasionally in mid traffic for no apparent reason, yet the recalled part that caused this has been replaced. Dealer offers no explanation and says car is fine.