I used to get a wobble at 55mph, had to be very careful of 'ruts' in the road made by wagons as this would set it off.
I changed rear bushes, convinced it was rear wheel steer.
I changed front bushes and repaired a slightly elongated hole in the panhard rod bracket on the axle end.
New steering damper.
All the above helped but didn't cure it, the thing that did stop the wobble completely was resetting the preload on the front hubs, I had practically none.
I can now reach 70-80mph in full confidence...But still drive about at 55-60mph.