Vibration At 70 Mph

2005 GT Ragtop with 47K miles has some vibration at 70 MPH.

At all speeds from zero to 65, the car runs perfectly smooth and straight... no alignment issues.
Once I hit 70mph the car has a slight vibration... After I accelerate over 70 (and beyond......) it's completely gone.

The rims are the AM Bullitt (made in China). My local tire/repair shop who mounted/balanced stated he didn't get a perfect balance from these rims.

Q1.) Do you think this is just a balance issue or is there something more sinister?

The suspension is stock, and I have future plans on replacing it (Koni Struts/Shocks, SR Springs and new upper/lower control arms front-rear all from AM), but when I hit a bump (doing normal street speeds of 35 mph) the car travels or jumps a bit.

Q2.) Is this because the suspension is just old and needs to be replace?

I've notice the converties don't feel as stable as the coups, I'm just wondering if it's time to give the suspension a refresh.

Pete, I agree... I have the car scheduled to go back to the dealer to have them take a look at it. It's under warranty so if there is anything more, it should be covered.

• "If it were a wheel/tire balance issue, it wouldn't go away, it would present at all speeds above a certain point.If it were a wheel/tire balance issue, it wouldn't go away, it would present at all speeds above a certain point."

> I don't entirely agree with that. My 08 Malibu did the same thing after I put new tires on. I brought it back and they rebalanced it. They could even tell me based upon my description if it was rear or fronts out of balance.

• "Is the car stock?"

> Yes, except for the Borla ATAK cat back and AM Rims.

• "Convertibles have much much flexibility in their chassis because coupes use the roof to triangulate their structural integrity. Think of the roof acting like a strut tower brace, but for the chassis not the struts."

> I can agree with that

• Also, what sort of vibration? >>>> t's more like a vibration/shimmy. When I look in the mirrors, they're vibrating
• Where and how do you feel it? >>>> I can feel a slight shake in my butt, and in the wheel.
  
• Is it only upon acceleration or upon coasting and deceleration, as well? >>>> No
  
• Can you cruise at that perfect speed and the vibration stays? Yes, if I go over or under that speed, it goes away. It's mainly between 65 and 75 mph.

Did a ride a long with Ford tech yesterday and wasn't able to reproduce any issues.
He said it feels fine... both going over bumps and taking it on the highway. At 70 there was nothing.
He did say my tires were over inflated...