If it's convertibles only that have the problem, then I would suspect something in the suspension that can wear. You're feeling this while driving, not braking I assume? Things to check out are strut rod bushings, ball joints, CV joints, steering linkage, etc.

It's possible, I suppose, that it's a warped rotor, and it's easy enough and cheap enough to replace these yourself. If it's a rotor, I expect if you apply the brakes while at the high speed, the vibration would increase immediately (and diminish as speed diminishes). In this case, if you feel the vicration in the sterring wheel then the front brakes are the likely culprit.

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David \\ (98 S70 T5SE Black, misc mods (mostly lighting), red calipers) (92 940GLE)