Fighting tje same issue on my 02 GTP. 4 new wheel bearings, new axles, new brakes and rotors, new struts and new tires. Last time i had the tires balanced tje tech found a bent wherl, and i replaced it. Was smooth as glass until daughter had car for 3 weeks, now same vibration that you describe is back again. Diesnt come on until 65 mph and up. Going to have the tires rebalanced this weekend.

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