2002 Ford Fusion handling feels unstable

Recently my, previously rock solid, Focus started to feel unstable while driving. It's not that noticable at low speeds, but at 50 and above it doesn't feel as if it's holding the road very well. When going straight it feels as though it's being buffeted by wind. If I change lanes on the motorway I feel as if I'm not quite in control. It seems to corner ok at low speeds, but I don't feel confident going round bends at higher speeds...it feels at though it's going to start skidding.

I've had it into a Ford garage, who've checked the tracking, tyre pressures, steering alignment and suspension. They say they can't find anything wrong. Does anyone have any suggestions?