The Project Pad

So I am looking into buying an '87 5000. The ad states that the car will run for about an hour, then stall out. My question is without and further info, what are the most likely causes? Said 5000 may become the daily soon if it seems to be something small (I know this is relative).

thanks
Ben

Does it restart? Perhaps the alternator isn't working and it runs until the battery dies? Goodluck!