Tires out of balance or what?

[hsmith]

My 1984 190d at about 40mph started to shake a little bit - you know, like the tires are out of balance. I took her to have the tires rotated and balanced. The slight shake is still there, but at different speeds - 55, 45, 60, etc. Now, while I'm driving, say at 55 she shakes a little. I slow down and get back up to 55 and she's as smooth as silk. I guess what I'm trying to say is that some times she will, and sometimes she wont. When I had the work done I just mentioned I took her back because I thought they did not get the tires in balance. So, they did it again. If its not the tires, what in the world could be causing this??

Sincerely,

Hugh