??Front end 'SHIMMY' - NEWLY DEVELOPED - ????

Just this week my front end has started to shimmy - like 'warped rotors" except this happens when I am 20-45 mph AND NOT APPLYING THE BRAKES??, 38 lbs pressure stock rims - decent tread!! HELP - AND IDEAS????
1990 SC 5 Spd Red/Grey - A TREAT!!

you coulda have a bad alignment..also check if theres any material ie dirt or mud behind ure wheels that may be causing it