[thenorm]

This problem sounds like what the passive rear steering does.

passive rear steering adds toe in when the spring is compressed. If you hit a significant bump or frost heave, your suspension will travel alot and you get a lot more toe suddenly, this can have the effect of shifting the car side to side. this should be more noticable on wet pavement (lower traction)

to the OP, you siad u had a 4 wheel alignment. what exactly was the specs the aligned to? (what was the rear toe?) the spec for a focus is too broad. ie. it can be "on spec" and still be quite screwed up.

what im getting at is: if you have too much rear toe-in, and hit a bump that gives you a lot more toe, then the car gets squirely. i would recomend a maximum of 1/8" combined rear toe in. minimum 0 rear toe. this way when u hit a bump, the toe added via passive rear steer is not outside of "on spec"