I feel a shimmy when I slow down/brake, sometimes

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Hi All,
 
I have a 1993 Audi 90 which I bought last September. Aside from needing
to replace the ignition switch and fix a small exhaust leak, it's been a
dream of a car.
 
However, I recently noticed that sometimes when I brake or take my foot
off the gas pedal, I feel a shimmy in my steering wheel.  Usually, as
soon as I hit the gas, it goes away, but when I brake the next time, it
usually shimmies again.  The weird thing is, it doesn't always do this,
just occasionally.  Also, I first noticed this about a week after I got
two new tires(Goodyear)and had them placed on the front and my old
tires(which look to be in great shape)placed in the rear.  Could one of
my new tires be bad?  If so, I'd think I'd feel it all the time.  I'd
also thought it might be a warped rotor but again, shouldn't I feel this
all the time?  I thought maybe it was out of alignment but on my old
cars, when they needed aligned, I'd feel a shimmy when I went fast, not
slow.
 
Does anyone have any thoughts/advice on what the problem could be?  I
really appreciate your help.
 
Thanks!
 
Tony