1990 Miata with 1.6 with shimmy in possibly the driveshaft.

I have a '90 Miata with a 1.6, I have only had it for about a
month, I have started driving it on a regular basis to work and noticed
a shimmy feeling in the car, I took it and had the wheels balanced and
rotated over the weekend, but as I was coming in to work today, I
noticed the shimming is still there, to describe the shimming goes
something like this, at about 60-70 mph, the car vibrates for a time
then stops for a time, it's like the driveshaft is unbalanced, after
the wheels were balanced, the vibration in the car has stopped but the
shimming has continued... does anyone know what the shimming may be...
thanks for the help... Trent