According to the specs your monitor supports a video bandwidth of 180 MHz. Maybe you just bought a cheap cable which doesn't support such a high bandwidth. You could lower the vertical frequency and see if the quality improves. If this is the case, it is certainly a cable bandwidth problem.
Test with a dark square on a white surface. Is one side red and the other blue, or are the sides just grey and look unsharp? Color shift might be caused by your new cable. Color shift can be corrected on some larger monitors, but I don't know if your monitor supports it. If the edges are grey and not colored red or blue, it is most likely the cable quality. A ferrite ring as mentioned before might not help in that case because it is an extra dampening filter.
Also did you reposition your monitor after connecting the new cable? We used 20/21" Hewlett Packard monitors ns in the past and when turning the monitor we had to readjust the compensation because of another direction of the earth magnetic field.