Re: front tires and their effects on wobbling at "high" speeds
When I build a wheel I use dial indicators. On my wheels I can true them within .003 TIR from side to side and the hub centered in the rim .005 TIR. The tire needs to be seated and straight also. Some tires will fit and be quite straight and others will not. Something as simple as a wheel reflector can cause the bike to shake at higher speeds. If you are using wheel reflectors it may help to place them directly opposite the valve stem. Weight in front of the handlebars can also cause this. An imbalance can cause the bicycle to shake violently at certain speeds. If this happens it can cause you to be startled. You can stop this by placing one hand or your nee against the top tube of the bicycle. Just remember the speed that it starts to happen also. There are different theories and discussions on what causes this.
I purchased my wife a rather expensive MTB the week we were married. I had ordered it and the shop let me assemble it also. The front wheel had come in with shipping damage and we had replaced it with the front wheel of a larger framed bicycle of the same model. The wife and I went for a ride up a canyon and on the way back down her bicycle started violently shaking. Her screaming clued me into the situation. I came up next to her and grabbed the top tube and coaxed her to slow down. I and the owner had looked over it quite carefully and had seen no other damage but it had to be something with the fork or frame. He had another one of the same size airmailed in and we assembled it and the new one was fine going down hill at speed.