Are you using radial tires? Is your alignment for the original bias-ply tires? Or vice versa?
Be sure there are no broken belts in the tires.
Try rotating the tires and making even pressure.
Inspect your springs, saddles, and shocks.
Remove the steering box and have it professionally rebuilt. Inspect and replace any other worn steering components. Inspect and replace any worn wheel bearings or spindles. Inspect and replace any worn strut rod or control arm bushings or ball joints.
Check the frame for square in each dimensional axis.
Be sure there are no broken belts in the tires.
Try rotating the tires and making even pressure.
Inspect your springs, saddles, and shocks.
Remove the steering box and have it professionally rebuilt. Inspect and replace any other worn steering components. Inspect and replace any worn wheel bearings or spindles. Inspect and replace any worn strut rod or control arm bushings or ball joints.
Check the frame for square in each dimensional axis.