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My guess: check the grounds, belts, wiring connections, and the alternator.
I don't know your car well enough to be certain, but most contemporary cars have things like the tach being fed via the ECU. When the voltage drops too low, the ECU, depending on the type, can go into an ether-land like state where it does all types of wierd things until the voltage either drops down low enough for it to completely shut off (usually stopping the car), or it comes back up high enough for the ECU to run in a normal state. I'm guessing your voltage dropped because you also stated seeing the lights on the dash dim, and they usually aren't computer controlled on all but the newest cars.
When a battery gets drained, it starts to develop a high internal resistance. What this means is that internally 12 volts may be generated by the battery, but at the terminals the voltage can read substantially lower. When this happens, if the battery is left unloaded for a period of time (like waiting for your wife at a store), the internal resistance will drop and the normal voltage will appear at the terminals again. I think this is what happened to your car, and I've listed what I think are the most common causes at the top of this post.
In most cases, the alternator should be capable of supplying enough power to keep the car running even without a battery once it's started, so even though the battery may be a lemmon, I would look at the charging system, grounds, etc. first as the primary culprit.
Hope this helped,
Bill Wagner
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