Heelllllpppppppp!!!!!!!

I was wondering if anybody could help me with my firebird running problem...ok its like this I recently changed out my 305 motor for a 350 switched the ecm and everything...now the car runs good when started but if its taken on a 30 or so minute trip and u shut it off and go inside the store or whatever...come back out and start it (starts fine again) but then dies...what gives? :banghead:

Are their any simple solutions i can try...just for referance...check engine light does not come on during these occurances. but does come on occantionally

does anyone else at least think it might be the o2 sensor over adjusting my fuel?