Bad vibrations

[Bill]

My son just bought a '92 Classic 5 spd with 188K. He just had the passenger
side motor mount replaced about a month ago. Today he was going about 60 or 70
and the whole car started to vibrate to where he couldn't see out of the rear
mirror. It lasted about 2 minutes then disappeared. Could it have something to
do with the mount or maybe another mount? He said he couldn't tell whether it
was in the steering, motor, or rear. Any ideas? With that many miles is there
something that consistently goes bad to make it do that? Thanks in advance.

Bill Shafer Valley City, OH  91 Classic