I have 55 Monterrey with the WW bias ply tires.  I have been driving on surface streets since I bought the car 50 mph and less.  Yesterday I took it out on the highway at 75-80.  It drives nice but shimmies with every bump.  Sometimes it feels like the backend wants to break loose even going straight.  We have some rough roads here so you hit a lot of ripples bumps and grooves.

What are your opinions, is it the bias ply ties that cause the shimmy or suspension issues?  It does not bounce like a bad shock and the shimmy is only for a second when the road is has a groove or bump.

I thought at the time it could be bias ply or just skinny tires, I think they are 710X15

Thanks