While I was driving on the interstate around Denver after driving for about 12 hours when my car started to get a little wobble that felt like it was coming from the rear of the car.  It feels like the back is kind of oscillating back and forth slightly.  It happened a few times for several seconds each time, some times only for a few seconds, other times for about 30 seconds.  At the time I was driving about 75-80 mph and it went away when I slowed down.  I stopped at friends house for a couple hours and then drove back home on the interstate with no problems. 
  
The same type of wobble happened back in March after driving about 2 hours, only it was more severe and at slower speeds.  On that day I had to drive about 25 miles back home going about 30mph otherwise it started to wobble.  I parked the car for a few days and took it to a shop, but the wobble had disappeared.  I haven't had a similar recurrence until the other day. 
  
Anyway, the shop couldn't find anything, I had the tires rotated  and in July I replaced the struts and got it aligned. 
  
Any ideas as to what could cause a wobble like this and why it is so infrequent? 
  
Thanks  
Will