If you are using NiMH batteries that is a large part of the issue. They run at a voltage lower than what Spektrum likes and it will cut the range. Or at least that is the case I have seen on the newer models that have 4 batteries in them.
I use NiZN in my setup and can drive the car to point that it difficult to tell which way it is pointed unless it is moving.
If you were having brown out issues on the receiver it would not be limited to the edge of your range, it would happen all the time.