Hey KKIM. I was browsing bridgestone's website and found that PDF with tire information..and I think this relates to your problem
http://mc.bridgestone.co.jp/en/produ...e/on_bias.html
Shimmy:
Left and right vibration of the steering assembly while driving in a straight line.
Low speed shimmy
Occurs when coastin down from around 80 km/h, peaks around 45-50 km/h. (vibration frequency: 3-5 hz).
High-speed shimmy
Occurs at around 110-120 km/h, the magnitude is small (vibration frequency: 10-15 hz)
Cause
A phase difference between the corning(cornering?) force and the self aligning torque. (That goes over my head but you can research into it, lol).
Tire related factors
Wear stage, inflaction pressure (high inflation pressure is better as it reduces the contact area), tread gauge, tread compound characteristic, casing stiffness, cord angle, etc.
IMO it looks like a tire related problem...