OK, this is a new one to me. I have what I think is a wheel balance related vibration in my MY02, but it manifests itself in a strange way. I

 

Thank you for a most comprehensive description of your symtoms. I wish others would be as detailed.
Yes, I have had this sort of "on again, off again, resonance" sort of vibration (on other cars). Since you stated you recently had new tires put on, I would suggest you focus your attention here. It is more likely that a tire will go out of balance within the first 500 miles of use due to "breaking in" the tire. The tire could have slipped slightly on the wheel in that time and sometimes, wearing off the first few hundred miles of rubber can cause the original balance to go off. It is also possible to have lost a wheel weight.
Also, there are those cases where the tire itself is a "lemon". It has a factory defect within its construction that may be very difficult to balance out.
Additionally, not all tire balancers are created equal. A wheel/tire is balanced on two planes - one being the obvious from one part of the rim to the other side of the "dish", the other one being from the outside to the inside. Not all balancers will do both. I'm not saying this is what is wrong in your case, but it might be. I'd start with a rebalance and go from there. Always get a "dynamic" (spin) balance on a machine that can check both planes.

 

Thanks, for the help, xviper....glad to hear someone else has experienced these kind of symptoms before. I've researched a Tire Rack preferred installer in my area who seems to have good reviews and road force balancing equipment, so I'll be making that my first stop.