<VV> Cowl shake on EM convertible

[Robert Hawley]

Howdy all,
    After having my car in Group Red for over a year, I am finally able to drive and enjoy it for the waning months of summer.  I have noticed that between the indicated speeds of 47 - 52MPH the cowl/windshield on my 1963 Spyder does the shimmy-thing: above and below those speeds it feels solid.  The shimmy is not violent, nor does it feel as if I might lose control, but is noticeable.  When the top is up and secured this does not happen. 
    My car has just had the entire front and rear suspensions rebuilt by Ken Hand in Pontiac, MI: new bushings, shocks, tires, springs, engine/transmission mounts, alignment, crossmembers (front and rear), steering box, idler, etc.  The new radial tires (185/80x13 running at 25#F -32#R -  P.S.I.) were balanced and there is no indication of tire imbalance.  (Yes, the wheels are on tight - I checked the lug nuts)  I have changed hubcaps and there is no difference. There is no structural rust anywhere on the car. I am wondering if it is harmful or if this is just a Corvair conv "thing"that happens to other drivers?  Is there a solution to this problem or just the nature of the beast?  Thanks in advance, Bob Hawley (CORSA member)