[OAC_Sparky]

Did you replace the belt and check the tensioner? That would be my first place to start. If the belt is stretched or the tensioner is seized then the belt will slip at high RPM. Check the belt (cracks and rips -- in case you mis-installed it when the alternator was replaced) and make sure it's tight, and pry the tensioner (like you were installing a new belt); it shouldn't be too easy to turn it, If it's floppy it needs to be replaced.

Apart from that, it could be a bad voltage regulator, which (IIRC) is built into your alternator. The only way to know for sure is with a multimeter.

Edit: One possible reason the car died with the AC on is because when the AC compressor kicks on it puts more load on the belt making it slip more and making the alternator produce less power.