If the wiring is still original there will be a pink 'resistor' wire to the coil in a ballasted system.

How do I tell if it's a ballast system?

The points are relatively new, the condensor is 20 years old, but less than 2,000 miles.  I thought condensers were either good or bad, and not intermittent?  How can I test the condenser?

I've timed statically to 10 degrees BTDC, but have not set timing dynamically yet.

What voltage you measure on coil (+) will depend on whether or not you have a ballast ignition system and whether the points are closed or open.  

Coils are not cheap.  Save their replacement for last.  Assuming you have a points distributor, replace the points and condenser first.  Then set the ignition timing.  If the problem persists, then replace the coil.

I'm still having intermittent problems.  The engine backfires through the carb, the tachometer needle swings wildly even though the engine speed doesn't change, and the engine has no power under load and will die.  I've ruled out just about everything, and I'm left with a suspicion of the coil because when I ground the negative lead on my multimeter and touch the positive lead to the positive terminal on the coil (ignition turned on and all wiring in place), my multimeter reads about 10.8 volts.  However, when I remove the wire from the positive terminal of the coil and touch the multimeter positive lead to the wire, it reads 12.3 volts.  This doesn't make sense to me.

The resistance between the positive and negative coil terminals is measuring 3.3 ohms, and the resistance between high tension and positive terminal is 9.6k ohms.  I think these are in line with the norm.

Does the above info point to a bad coil, or are there other tests that need to be carried out?

Thanks.

Sounds like it might be running a bit lean. Check carburetor oil. Top off in necessary. Richen mixture a bit. 

Thanks everyone for all of the suggestions.  I will follow them and report back.

Limited details to what engine/distributor you have but change the condensor (if you have one fitted).

Fuel problems don't typically show them self with jerky tach movements so this suggests a possible ignition fault.  Start with the less expensive items as Dan suggested (check the cap, rotor, plugs, plug wires, clean/dress and set the points, and check all the low tension connections to the coil).  After you have gone over the basic ignition parts, at least check and reset the timing if necessary.  If all that fails to correct the problem, BORROW a known good ignition coil.  If the problem goes away with the borrowed coil, buy and fit a replacement one.

Your profile doesn't say where you are located. In upper North America we've had a cold spell, and if your weathre is colder than the Mini is used to, it is going to be grumpy.

Check that the manifold bolts and the nuts holding the carb on are tight and that all the hose connections are also tight-fitting.

Have you checked the condition of the points and the dwell? If you have a dwell meter, it will give you better results than feeleer guages. It will also indicate consistency of dwell- if the needle moves you'd have erratic spark. Check the dizzy cap for carbon tracking inside or cracks? The rotor? Deeper in, check the dizzy shaft to see if it has any wobble from worn bearings.

There're a few missing details in your question on what car and what tach.

I do have a vacuum advance distributor, but I've pulled off each end of the hose connecting it to the carburetor one at a time and sucked/blew air through the hose and doesn't seem to be leaking.  I have new gaskets from the carb to the manifold and haven't recently removed the intake or exhaust manifolds from the head.  Any suggestions on where to start?

Fluctuating idle and occasional stalling sounds like a vacuum leak.

The needle moves, but not smoothly.

The last time I drove it, it died on me a few times and I was lucky to get it restarted and back home.  

What behavior do you see on the tach when you blip the throttle and the backfire happens?

How does the engine behave during acceleration under load (like accelerating uphill)?

My classic mini's tachometer bounces a little while idling and the RPM's increase and decrease slightly on their own.  There is also an occasional backfire through the carburetor when I blip the throttle.  I'm thinking electrical, so I checked for shorts/wiring breaks in the wiring to/from tach, distributor and coil.  Anyone have ideas on what the problem might be?