My 2002 Boxster S, I have two different set of tires, one set for summer and the other set for winter.... YES winter. I always have my Porsche dealer balance my tires. Not only they spin balance them but they do a "LOAD" balance as well. At the sametime I have them do a 4 wheel alignment check as well.
My car did the samething as yours. The dealer technician found that 3 out of 4 needed balancing. If I can not hit 155 to 160 miles-per-hour within a mile or two, then there is something wrong.
:-D
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