I have an 86 rx7 that is automatic, when I get out to about 40 mph the car starts to shake but when I switch it into overdrive the car smooth out again. I was thinking that it might be something with the transmission. I had the transmission fluid changed and the filter as well. The tran is a used one that I had put in about two years ago. I was thinking that having the transmission flushed would solve my problems? Has anyone out there had a problem like this before?

 

I've had that problem with my manual.

You might not have a hydrolics/gear problem so much as the crossmember that holds the auto tranny in is loose or its booting might have disintegrated. Even loose differential mounts or damaged universal joints can cause vibrations at variable speeds.