Sounds like a steering gearbox adjustment problem to me. I overtightened mine once many years ago. Result was a car that wouldn't quite go straight. It was always just a little bit to the left or a little bit to the right. Very annoying.
Simple to cure - I added a bit of slack to the steering. Just loosen the locknut on the steering box and turn the allen head bolt *in* a little bit - start with perhaps a quarter turn. See if this improves things. Five minute job on your 123 diesel.