'92 SE-R Vibration Problem

[L. Boody]

I have a 1992 SE-R with 160,000 miles.  Over
the past two years it has developed a vibration
from the engine compartment.  It is most noticeable
when the engine is cold and can be described as
almost like a low-frequency rumble.  As the engine
warms up it is still apparent but it is not as bad.
It sounds like something is out of balance.  It
is also apparent at while driving but not as
apparent as when it's at idle.

When I turn off the engine, the engine seems to stop
very suddenly instead of making a few more turns
from momentum.  (sorry, I'm no mechanic)

The engine revs smoothly and still pulls strong.
The tach is rock-steady.  My mileage is still the
same as always(average 30).  The distributor/
rotor are new as is the PCV valve.

Instinct tells me that it could be the engine
mount bushings gone bad but that seems too
simple.

I want to keep this car going as long as I can.
Any suggestions would be appreciated.