Is my altenator going bad? Is it the voltage regulator in the altenator that is failing?

CAR = 94 GSR with 115,000 miles

PROBLEM IN THE BEGINING = cranks but does not turn over the first try. It will turn over the second time. This is an intermitent problem.

PROBLEM NOW = while driving and at idle, the tach needle drops below 500 rpm and then

jumps up over 550 rpm, In addition, the headlights are dim the brighten up when reving the car at idle.

Recently at a stop light the car sputtered and died. It restarted after the first turn of the key. Then it sputtered and died a 2nd time at another stop light and restarted after the first turn of the key.
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- I tested the battery with a voltmeter and I get 12.78 volts with the car off. I tested the altenator by connecting the voltmeter to the battery and with the car idling I get 14.63 volts.
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- Altenator, starter, fuel pump, fuel pressure regulator have never been replaced.

- The battery is 2 years old.

- I have replaced: coil, ignitor, main relay = these parts failed awhile ago

- Distributor cap and rotor, sparkplugs, ignition wires have been replaced as routine maintenance.

- Fuel filter = just changed it.

- Fuel Pump = has not been replaced but I hear it prime when I turn the key to the 1st position.