I've hauled my car down from some pretty hairy speeds and its straight and solid all the time. This may seem condescending (its not meant to be) so feel free to virtually slap me, but are you braking in a straight line. If you brake hard and move the steering wheel or brake in a turn, even a slight one, you will shift the weight of the car around the axles which will give you the snakey feeling.
Advanced driving courses teach you how to manipulate weight transfer to your advantage and a feel for the movement of the car.
Of course it could just be a sh!t road
