Yes, it happens when the car is stationary.

Yes, the vibration is there after 3200 rpm, it just seems to peak in
amplitude there.

I immediately suspected an electrical problem of some sort because of
the intermittent nature, but the service advisor said that the "check
engine" light would come on if one of the spark plugs was not firing.

No, it's the 2.3L non-turbo and there is no mention of the DI with this
model.

Thanks for your comments (and Graham, too).  I get the impression that
the vibration would be far worse if one cylinder was not firing at all,
but I will investigate further.  There doesn't seem to be any loss of
power and the car is perfectly drivable -- it just vibrates.

Any other thoughts, anyone?

In article <46fbe943$0$8415$db0fefd9@news.zen.co.uk>,

[snip]

Okay, I'll try that when I get a chance.

I haven't noticed any lack of power.

Agreed and there is no correlation with acceleration or turning.

I don't know.

No points so I guess that means electronic ignition.  Nothing too fancy.

It was certainly my first thought, but how could an exhaust problem be
intermittent?

No, although I can't rev the engine with my head under the hood/bonnet.

I haven't pulled the plugs because I don't have the deep socket I would
need, but they were changed 18k miles ago.  No lack of power.

Thank you, Charles, for sticking with me on this.

I'll list my observations being as specific as I can.  I know there's
nothing like actually experiencing the problem first hand.

1.  1995 900S, 2.3l non-turbo, 4 cyl., 113k miles.  I've owned the car
for all but 8k of those miles so I'm very familiar with the feel of it.  
Normally, it is smooth.

2.  Went on a long trip (375 miles there, visited a few days, 375 miles
home).  Next time I drove the car (short trip to store evening of
return) the extra vibration was present for the whole trip -- from
starting of the engine to shut down both there and back.

3.  Next day, no extra vibration at all.

4.  Next day, extra vibration present for all driving that day.

5.  Extra vibration gone for a while -- I thought (hoped) forever.

6.  It progressed into an intermittent problem.  Now it comes and goes
during a trip -- perhaps several times or it might not happen at all or
it might happen for the whole trip.  These are just short, around-town,
trips, 10-30 minutes.

7.  Description of vibration: It feels and sounds a bit like a hole in
the exhaust -- low pitch, feel it on the gear shift and gas pedal, don't
feel it on the steering wheel.  A passenger not touching anything might
not notice it at all.  An exhaust problem I would expect to vary in
amplitude with engine throughput -- more gas, more vibration.  This is
*not* what I observe.  Vibration seems to be independent of engine
throughput, only dependent on engine speed.  It happens when the car is
stationary and is exactly the same as when the car is moving.  It seems
to increase in amplitude as engine revs increase, up to about 3200 rpm
where it either peaks or at least levels off.

The vibration is not random feeling.  What I mean is that if the engine
was misfiring randomly -- random sparks not occurring, say -- you would
notice that randomness in the feel, it would feel jerky.  That is not
what happens.  This vibration is very regular.  It feels just like the
normal engine vibration, but just at a greater amplitude.  Does that
make sense?

That's about the best I can do.  If you think of anything let me know,
but I may just have to see if it gets worse or hope I can demonstrate it
to a mechanic.  The local dealer is about 25 minutes away so by the time
I get there it may not be happening any more.  I'd hate to have to leave
it with them for a few days for such a minor problem.  The car is quite
drivable after all.  But I worry that the vibration is indicative of
something potentially serious.  Besides, it's just annoying!

Thanks, again.