Tim - try replacing the IAC motor. What? the Idle Air Control motor, a
little cylindrical guy on the throttle body with 3 or 4 wires going to
it. They get old and sticky and just quit sometimes.

Fugit - If the serpentine belt tensioner is at the end of its travel,
get a new belt. If not, take the alternator back, most reputable parts
stores will have at least a 90 day warranty. Hope you saved the receipt.
 Oh, yes the voltage regulator is an integral part of the alternator.