My

. First I have a question, was it like that before you changed the ball joints and tie rods?? About 6 months ago I replaced the ball joints on my car and they were extremely tight. Tight to tilt the stub and obviously tight to turn the stub in the housing. That caused the wheels not to return to the straight position when the front was in the air. This should be able to be achieved by the self-centering springs inside the rack.
You have new tie rods which will add to the friction.
It may be that a bump in the road turns the wheel slightly to a side and then it is too tight to return because of the ball joint friction.
If the tie rods are disconnected and the steering wheel is turned max to one side and you let it go, it should return to center with a speed. If not, then the rack is dry as old grease does go dry and that can be added to your problem. I do modify my steering racks even further. Above the pinion is a little piston with a V in it. The V pushes the rack down to prevent play. That tension can add to the issue. My mod is to remove the spring that pushes down and replace it with a spacer that is machined to the exact size so that the rack doesn't have a play but there is no pressure on it either. The centralizing springs remain as is.
I disagree slightly with Schlitz in that the centering of the wheels is a combination of the self-centering springs in the rack and the castor. I also run my race car and streetcar with 1 - 2 mm toe-in. My theory behind this is for the wide rims that have a slight leverage to pull the wheels out due to the tyre friction on the road and then it almost straighten them out to 0 mm. I must add that I don't use Mecapart's wishbone bushes. I make my own from Graphite impregnated nylon. The brand name in NZ is Nylatrol. This eliminates all play due to the rubber in the bought ones. The only rubber in the streetcar is in the tierod ends, but the race car is metal to metal as seen in Mecaparts catalog.
Regards Frans.