I had some work done on my dash last week (71z) and now the speedometer needle is really unsteady; the needle fluctuates approx plus-minus 3-4 mph at times. Any idea what causes this? It's very annoying.

Most likely the speedometer cable didn't get put back in at exactly the same angle it was originally. You can reach under the dash and try to move it a little and see if you can find a better position where the turns follow a larger radius. If that doesn't do it, you can usually fix this by getting a new cable and installing it taking care to keep the bends as wide as possible.

i just changed the speedo gear in mine... now the needle fluxwates alot and goes to zero when i have my foot on the clutch or when i am not accelerating.

You buys either have a cable that is not seated correctly, or is broken. All that has to break is the tip for any problems to appear.