1997 Chevy Astro

Sometimes I will be driveing and the van will die, the last couple times it happened I looked at the guages and everything is "off" but if I put it in neutral in time and tap the gas pedal evrything comes back on and stays running (if i'm not quick enough then I have to restart it but it starts with no problem)
i have had the alternater and battery checked at autozone, they gave me a print out and it said everything was fine except battery was a little week. I have replaced the alternator 2 x's in the last 3 years.
sometimes the dash electrical guage jumps but stays between 13.5 - 14.5. I have seen it drop down way low a few times but then comes right back up to normal. Could this be caused by a short somewhere, I just think the alternater should last more than 1 year so something else could be wrong. Any help would be appreciated cause it never does this when a mechanic is looking at it. Im lost here