Does it always do this or are certain roads worse than others? If you aren't sure drive around and test it. The road surface itself can cause this.
Any uneven wear on the tire tread can do it. The second set of tires may not be much better than the first. Unless you know what to look for, some kinds of wear are hard to see.
Any worn bushing, ball joint, or other suspension component can cause it.
A loose steering column is definitely a problem and needs to be fixed ASAP.
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11-Sept-01, The day the world as we knew it ended.