Bad Vibrations

[RandallNeighbour]

Hey, I did have one question for all you fellow boxster owners. I am now 1/3 of the way through my vacation, driving from Houston, TX to Northern CA and back.

I've had HOURS to ponder why the rear of my car vibrates a lot from time to time, then is smooth as silk like I was driving on glass. Same goes for the front end. It either shivers like I have a bent rim, a bad tire, etc. Some times the steering wheel shakes, and some times it's nice and steady. All this happens while it's set on various speeds on cruise control. So speed is not the issue here (ie, vibrations come and go at 65, 70, 75, 80, and 85).

Just to clarify, I can be driving along on seemingly smooth black top on cruise control and the shimmer begins, then fades to nothing, then comes back, etc. I am at the same speed when this happens.

Before I left, I had a four wheel alignment done as well as road force balancing on all the tires. But, this problem has been in existence since I bought the car a year ago. I just can't seem to fix the problem. Replaced the tires, had them balanced four times, had it aligned four times too. Grrrr.

If this were a bad tire, a bent rim, a tire out of balance, it would be a consistent issue at certain speeds, right? Am I just feeling crappy pavement below my butt or can someone tell me what's wrong with my boxster so I can have it looked at when I get into LA?