Over-tightened and worn steering bearings can cause issues with tracking. Actually, one or the other can do the same. It's worth it to loosen the stem nut and give that a go, nothing to lose. I know that on my road race bikes when I have the stem bearings too tight I get a "figure-eight" action, where I have to over-correct (and over-correct, and over-correct, and...) and that causes massive issues at low speeds.
Try having the bike on the stand and test the steering. Adjust the nut to get your favored results.
Also, check fork height. Jakobi touched on it, make sure your geometry is where you had it last.
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Phil
(New bike pending...)
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