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The first set of tires I put on my boxster had at least one out of round tire, and it caused the exact problems you are describing. I bought a set of Sumitomo HTRZII's and had them road-force balanced on a Hunter 9700 series machine and the problem went away... the car was smooth at any speed.
As others said, get that missing lug nut on the wheel and drive it again at 85 and see how it acts. If it vibrates, I'd say you should get the front wheels road-forced balanced. Some Discount Tire Company stores have the machine.
Then drive the car again. If this doesn't fix the problem, I'd say one or both of the front tires are out of round. There's no way to fix this that I know of.
My out of round tires were Kumho's, by the way ... another budget tire for our cars.
My third set of tires, required for larger rims I purchased at the beginning of this year, are Michelins. I spent the big bucks for two primary reasons:
1. I have far better wet traction and they're far less noisy than cheap tires as they've worn. My Kumho's got really loud after about 7,500 miles.
2. None of them are out of round, and I have zero vibration other than road feel response, which we need to drive safely at speed.
Hope this helps!
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