What you're describing is a feature that's common for electronic power steering. It sounds to me like you're used to older cars which use hydraulic assisted steering, where the steering is either loose or tight all the time. With electronic power steering the level of assist the electronic motor applies is proportional to the speed you're traveling.
You can test this out by going to a parking lot and driving around at a very slow speed (~10 mph). Do a few turns and notice how the steering wheel is very loose and easy to turn. Now drive off onto a main road and speed up a bit. As you get closer to 35+ mph, the steering wheel will gradually tighten and become more responsive. This is 100% completely normal with electronic power steering systems.
I can't explain the zig-zagging that you describe, because I don't know if you're naturally overcompensating for a steering system you're not used to. But I think a quick way to figure out whether it's really a problem or not is to visit your dealership and test drive another Focus (or another Ford model) to see if it really is the steering system. I don't want to discredit you entirely because maybe you really may be onto something. But I just figured I'd explain how newer steering systems work.
I can tell you right now that almost all cars that use electronic power steering have speed proportional assist. It took me a while for me to get used to it too when I bought the Rabbit. Sport trim cars (GTI, MazdaSpeed3, etc.) have their assist levels turned a little lower to emulate old school hydraulic power steering responsiveness (which is probably what you're more used to). I'd rather have that feeling too, but unfortunately these cars have steering that's tuned more for comfort rather than precision.
The good news is that the level of assist can be tweaked very easily. People who work for Ford can use a diagnostic tool hooked up to your car's computer to actually change the value of the level of assist (much like changing the mouse sensitivity on your computer). The bad news is that your dealership probably won't let you do it by request. Believe me I've tried to get them to set mine to a sportier setting (GTI's default value) and they refused even when I attempted to bribe them, stating liability reasons.