Flat spots can happen pretty quick on summer type tires and go away in about the time you mention.
Also make sure you "evenly" TQ your wheels starting at a very light TQ and work your way up. If I just burp my wheels on with an impact it shakes @ 65-75 every time. Take them back loose and TQ them very light the first time then to spec with no or little weight on the tire shake is gone.