Heh, quite the list of possibilities. I had this problem and checked all of the above. The tires were perfect, the balancing was perfect, the directional wheels were going the right way, the alignment was bang on... but at 115 km/hr it would start to vibrate. Got so that I was trained to accelerate to 110 and stay there without looking at the speedo. Then I replaced the stock struts at 70,000 km with Tokicos and went for a cruise -- when I glanced at the speedo I was doing 160 km/hr without realizing it! Glass smooth. The stock struts go bad fairly quickly and the effects can be subtle.