For what it's worth, my Focus does the exact same thing. I find it feels too heavy most of the time, and it springs back to center way too hard... except when going slow, when I actually have to force it back straight.
I had electric steering on my previous car (Rabbit), and it was about the only part of the car I actually quite liked. I agree the Fiesta's steering is actually excellent, too. The Focus just doesn't feel right... too heavy, and sometimes it's fighting really hard, sometimes it isn't doing a thing.
Good luck getting it resolved at all, though. I can see dealers refusing to ever admit there's anything wrong with it...