steering box adjustment

[pontiacbird]

I have a steering box of iroc z 1988 on my firebird 1968. when I make the adjustment with the top bolt, the direction is tight and direct. I like it a lot like that.the problem is that only a few kilometers later (5-10 kilometers) the direction becomes less tight and more blurred in the center. I have made the adjustment a few times in the last years and always the same result. I can not understand why it does not keep the tight fit as I would like. it seems to me that it can not be worn out in such a short time?