Strange Suspension problems

[hhudson@rimail.interlan.com (HUDSON, HEIDI)]

     Anyone who could help figure this out???
     
     I bought a set of new tires a week ago.  In process of attempting to 
     align the car, it was determined that the left front side could not be 
     aligned and required a trip to the frame shop.  (Although I bought the 
     car used, I've supposedly had it aligned twice with no problems.  The 
     ride was "rougher" than I was accustomed to, but I attributed it to 
     the quattro suspension).  
     
     I had the tires mounted and balanced and made an appointment for the 
     next day at the frame shop.  I drove the car home - approx. 15 miles.  
     On the ride home, the car began to vibrate badly at 60 mph on the 
     expressway.  I got off the next exit to check the tires, and it went 
     away as I slowed on the ramp.
     
     The next day frame shop adjusted the front end and I then had a 4 
     wheel alignment done.  The first day out of the shop it seemed great - 
     a much smoother ride.  Yesterday at 40 mph the car began to vibrate 
     again.   It continued for 1/8 mile until I stopped for a light.  As I 
     slowed down, the vibration sounded like a rhythmic thump related to 
     the turning of the tire.  It seemed to be coming from the left front.  
     When I started up again from the light, it was gone.
     
     Doesn't seem to happen at a particular speed and it goes away for no 
     apparent reason.  It's also very brief in duration.  It happened 
     before the frame shop touched the car, so I don't think it was 
     something they did.  The tires were the only change - could it be the 
     tires?  It never happened before these tires were installed.  What 
     could cause this that would spontaneously go away?
     
     I'm stumped - any suggestions?  Any help would be appreciated.  By the 
     way, its a 1988 90Q.
     
     TIA
     Heidi
     hhudson@rimail.interlan.com