Hi, i had the same problem on my 6745, the problem was compounded by the following:
1. Damper was out of oil
2. Play in all the rod ends (ball joints)
3. loose drop arm on steering box
4. Play in the front pivot pin.
5. front wheels were set on maximum track width
in-fact just adding oil to the damper and bleeding it stopped the problem on my next trip down the road,
however i currently have the whole front end off to fix the pivot pin bushes and am replacing all the ball joints with good used ones from the wreckers (the old CZ type with grease nipples seem to fair better than the crappy Italian sealed units on my tractor)
and i will re fit the front wheels to 2" narrower track when i re fit them, this will improve bump steer as the kingpin angle will not be completely inside the inner edge of the tyre like it is on the widest setting. and resetting the back wheels on maximum track width will make it more stable on hills