Vibration Need Help

[Walt92Conv]

I am looking at buying a 92 GT convertible.

In fifth gear there is a lot of vibration. If I shift back to fourth and maintain speed the vibration goes away. The owner says the motor mount is shot, and that a shop told him that it only affects fifth gear because the motor is loaded down at the slower speeds.

I will be driving the car again this afternoon and plan to see if going faster in fifth will make it go away. It does not vibrate under any other conditions as far as I can tell so far.

Can someone help? Does this sound like a motor mount or something more serious?

Thanks,
Walter