i had the same problems and it was the steering box. I had adjusted it too tightly to try and reduce the play, and it was fine for a while, but after a couple of weeks, it stopped auto-returning to centre, and became very stiff. New steering box was necessary, but not that easy to find tbh. Luckily, my local landy garage had one from removed from an almost new 90 in the late 80's that had been upgraded to power steering, so it'd barely done any work in its life, and just been sat on a shelf ever since! I felt like a bit of a cock though... lol