96 Eldorado voltage dropping

The voltage is dropping on my 96 Eldorado. The car has 91k and I have owned it for two years. I can tell the alternator has been replaced because it is cleaner than the rest of the engine. When I start the car the voltage jumps up to about 14.2 volts which is good. As I drive and the engine warms up the voltage begins to drop. When the car is hot and I turn on the accessories the voltage drops to about 12.9-13.2. I have seen it drop to 11.9a couple of times but that is rare. I just drove the car on a 1500 mile trip and never had any issue with a dead battery. When I turn the key on and before cranking the engine the voltage is 11.9. The car cranks slower that it should but not all that bad.

The symptoms are that as the alternator heats up the voltage drops. If I take it off the car and have it tested it will probably show that it is fine because it will be cold at the shop/store. Do I have a bad Diode or ? Any help would be appreciated, it looks like a job getting it out of the car.

Rebuilt components are always suspect because you don't know who rebuilt it or with what quality components. The voltages you indicate don't seem much at variance with those observed in my car. As to the slow cranking, is the battery the correct size for the car? How old is the battery?

when you take it off to have it bench-tested, tell the parts counterperson to run it a few times & see...

how's the belt condition/ tension as well?

The battery was purchased this spring and I did charge it before installing it, I guess it could be bad. The belt tension seems to be okay, no squealing. I had another used alternator lying around off a 94 Concours and installed it for the heck of it. I will take the other one in to have it tested later. I get almost the same results with this one but voltage is slightly better. I turned on every possible accessory to put a maximum load and here are my results. Headlights seem to drop it the most, brings it down to about 13.2. I then turned on both heated seats on high, rear defroster, air conditioning, wind shield wipers on high and the radio. With all these on and the car idling it will drop below 12 volts. If I rev the car up it does come up some but not much. Maybe this is normal. Thanks for the replies. I am putting the car up for sale and did not want to pass on a problem to someone else.