2000 silverado 5.3L. Truck turned over a little last night, but did not start. Battery tested bad, so I replaced it. After the replacement, it started right up, and i let it run for 10-15 minutes. Today, it didn't even turn over, just...."click". Left for 10 minutes, came back, and the voltmeter says 12.5V. WTF? I say....got back in the truck, and it started up, but the idle was low and it instantly died. I started it back up and revved it a bit and it was fine. Idle is at 500 rpm, oil pressure is between 40-60 depending on idle or revving, battery charging at 14-15 volts. Why did the truck not turn over this morning, but after 10 minutes, it works. I am confused. I thought it was a dead short that was draining the battery, but the battery after sitting all night checks out at 12.5V, and the dome light works and the remote unlock. I am lost. Help please.