Mine does this too, although it's just a slight vibration & only noticeable
if my hand is on the gearlever.

I don't think it should do it either but you're right, it's not the
transmission if it does it in every gear. It'll be the engine or torque tube
causing the vibration. Could be something to do with the balance shafts
maybe?

Malc.

suggestions:
1. check that the balance shafts are properly timed - as I recall, when the
balance shafts are out of time, it's right around that RPM where problems
occur
2. check your clutch - if you have the old rubber centered type, this could
be a sign of the rubber going away
3. check motor mounts - usually bad motor mounts cause worst vibration at
idle, but....
4. isolate to engine,not engine by holding down clutch, car stopped and in
gear - bring revs up - if it vibrates, it's not the clutch disk or drive
line components,that will be good to know.
5. check belts on front, like AC and PS belts
6. check ignition timing - did you slip a tooth on the belt?

my guess is #1, but it's a guess

Clutch rubber center is deteriorating.

Kevin
Catellus Engineering