You got a can o worms there, and everytime I talk about a 2.1, I get in
trouble.Never stopped me before. The o ring is the problem like described
above. Should be ez to do and avaliable from vendors like BD(I ordered
yesterday and need my parts,so there). I would wonder how this shaking mess
started? Did you "fix " something and then this mess started? I would look
in that area.  After that, look for vaccum leaks. Check the breather tower
base, attaching nuts(4).Check any hoses,and put hose clamps on questionable
connections. If your brave, use a propane torch(unlit,and AWAY from any
potential sparks) to "follow" any hoses or hose connections. When the
engine runs better, you found a leakBANG!If you are chicken(and you should
be), use spray water to check these.  The fuel injectors can have damaged o
rings. The throttle body may haved worked loose and is allowing  unmetered
air in. Just 2 phillips screws here that may need attention.The intake
runners bolt with 2 bolts near the injectors. Check em.One of mine was
wiggelable.Check the power brake hose that attaches to the air distributor.
I understand that the 2.1 idle stabilzer may need dissambely and
cleaning.Could be your AFM needing the tantallum capacitor fix or just
grounded.Check the big hose clamp that attaches the throttle body with the
rubber boot.The rubber boot may be cracked. use weather stripping cement to
fix this, or fork out(another) $90.

Hook up a vaccum gage(cheap) to a place south of the throttle plate. See
what you get.Sometimes a leak at one cylinder can be differentiated from a
leak from the air distributor, north to the AFM. I think a intermittant
drop in vaccum is a leak at a cyliner,like a plug misfiring.A very
low,steady reading is the latter(I think).

You may have  a combination of small leaks and a dirty stabilizer.I would
give the engine a good bath first with about 5 cans of gunk and a paint
brush.Dont touch the heads unless the vaccum gage tells you to.Good luck.
Let us know the progress(if any).