Between 2800 and 3200 RPM (50mph), third gear on my 97TA 6 speed w/30,000 miles makes intermitent vibrations.

It'll go rrrrr.......rrrrrr.......rrrrr.....rrrrrr about 4 times and then it'll be fine. All other gears are perfect and don't do this.

I bought the car used like this and it doesn't seem to affect performance at all but it is extremely irritatin.

Would a change to synthetic fluid help and why is this happening? It wouldnt' be the driveshaft, would it?