Car wont stay started

99 chevy malibu lt 3.1. The the battery and alternator are good. the wires and plugs changed, oil & oil filter changed. Lower intake manifold was just repaired and so was the heatercore (replaced). The car will turn over and start and immediately shut down within a second or 2. what's odd is when i got it back from the mechanic it started fine for 4 days, then bam this happens. He came back out, got in it and it started right up (murphys law) it ran for 4 days again then the same issue again, it wont stay started. The theft system light doesnt come on as this happens but, the behavior is very similar...plz help ty.