How 'bout yer shocks? How old are they?
Combine older shocks with a slightly out-o-balance tire/rim, and you'll get a pretty good bounce at that speed.
Is the steering wheel shaking left/right, or do you just feel a bad shake/vibration? If the steering is jerking back/forth, you got a bad rim/tire, and possibly a tie-rod end....
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Joel Adams
C3VR Lifetime Member #56
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"