What do you mean by "oversensitive"? My 2002 SVTF is rock-steady at speeds well beyond the posted freeway speeds; if you don't do 75-80 mph around here you're bound to get run-over by an Excursion or Suburban or 18-wheeler! Seriously, if you think something doesn't feel right then go to the dealer and get them to check all the alignment parameters and make sure it's all to spec.
My highway speeds range from 55 to 80 miles per hour. It just feel LOOSE, as if i don't have control over the car in the straight line. My second car is Civic SI and it is rock steady at the same speeds. Frankly that is the one thing I never liked about Focus, that and the SURGING ENGINE - that, "knock on the wood," was addressed by the new ECU update.
When ur car is just stopped at a light or something turn the wheel in both directions. When u go to change directions if u get a clunk sound at all or feel a clunk than take it into the dealer imediately. I had my entire rack and pinion replaced and i had the same problem ur having.
If this occurs on every road, check your tire pressures. On some sections of grooved roads, my Focus follows the grooves cut in the concrete and it's a bit annoying.
If the tire pressures are OK and it's not the grooves in the road, have the alignment checked. If that's within spec, there may be a mechanical problem or something loose. If there aren't any problems found, you can have the caster adjusted so that it isn't as sensitive.