E39s have notoriously hard-to-balance wheels, and the larger wheel/tire combo only exacerbates the problem. That said I can suggest exploring a couple of things:
1) I've read that a worn control arm bushing (particularly the large, pressed-in one in front lower control arm on the I-6) can cause a shimmy at certain speeds (check 540i6.com "Common Problems & Fixes")
2) Many aftermarket rims can be lug-centric, whereas OEM wheels are often hub-centric, resulting in impossible-to-balance rims. The only solution I know to this one is to purchase hub adapter rings to make your aftermarket rims essentially hub-centric. I've checked and I know that Summit Racing sells these, but you will need to know the O.D. of your car's hubs and the I.D. of each wheels' hub. Fortunately they're cheap, but not everyone owns vernier calipers for such fine measurements.
3) Also, make sure your hub and rim mating surfaces are completely cleaned off and rust-free, using brake cleaner and fine sandpaper. Then apply a fine film of CV joint or axle grease to your hub surface prior to re-mounting the rim. This will give you the best fit and facilitate removal of the rim next time.
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2006 Honda Odyssey EXL-RES
2006 Honda Accord V6 EX-L Sedan
1999 BMW 540i 6-Speed (Sold to M3UOND on 9/21/06)
1991 Honda Civic Si - Occasional Beater