My 97 Cavalier has had an intermittent braking problem since about a
month after purchase, about 2yrs ago.  The first time I had turned a
small curve, in a parking lot, and then pulled into a parking spot, the
minute I braked, the car became a bit jerky, and acted like it was
going to die, and I had no brake power.  This prob continued, but was
so infrequent, I just let it go, the engine light never came on, nor
did the abs light.  This was in good weather time.
About 3-6months ago, I had an auto mechanic replace some type of sensor
that he had to drop the transmission a bit in order to do so, but I
forget what he called this sensor, but this didnt solve the problem.
And, now it is winter, and after our first heavy snowfall, the car had
this jerky brake problem a lot on snow covered roads. It's scary
because it happens when you are stopping, and its hard to get the car
to stop totally when this occurs.....when this occurs the brake pedal
becomes hard, and one time I had to put the transmission into neutral
to fully stop the car.  No warning lights come on, on the dashboard,
nor does the engine stall/die. So, I am told with no warning lights
appearing the car will not throw off a code that can be used to
diagnose the problem.  The funny thing is last winter, I didnt notice
this problem, and if it did happen last winter it definetly didnt
happen as much as it happened this winter, thus far.

I have had the front brakes replaced, and maybe the rotors too, not
long after I bought the car, I believe.  I have had a tune up done, and
have the oil changed regularly.  The car has 109k miles on it and
overall is an excellent vehicle, but this problem is scary, especially
in winter time.

I need some good advice!!!