P0300 Codes are so ******' aggravating. It seems like any little thing will set one off and in turn leaves you scratching your head.
If you haven't already, I would check to see if your MAF is descreened and if so, be sure there isn't some type of debris stuck to the wires inside. A friend had this happen to his 98 T/A. It would start but almost immediately die. Once he found the problem, the car was back to its old self.
You could also try unplugging the MAF to see if the car runs better in Open Loop. If so, its possible the guys at Autozone were correct and your MAF went to ****. If not, then its time to look in another direction.
Good luck, man.