Another thought would be binding or bad bushiings/bearings in the swiveels or the U-joints.
Get the front end up in the air and see if the stiffness is still there. From there, work methodically back through it, using a U-joint tool (not the fork type, the bolt type they're only a tenner or two) to pop the drag links and tie rods loose. This way you can check components individually.
The other thing to check is the steering relay - if it's empty/rusted up inside that can play hob with your steering.
ajr