2001 Chevy Impala Engine hard to start and stalls while dri

My wife has a 2001 Chevy Impala LS 3.8L and the car ran great until recently that is. Doesn't matter if its early in the morning, after a short trip or long trip. The vehicle is very hard to start and this is not all the time, sometimes the vehicle starts on the first try and runs great but other times we sit for at least half an hour trying to get the vehicle stated. It tries to start but runs very rough and sputters out. Sometimes while driving under normal conditions 35-45 mph the vehicle just kills for no reason and is very hard to get started again. I recently changed the fuel pressure regulator but the problem persists. No codes come up on the computer and I hate to get rid of this car because of something simple. Help this frustrated chevy owner. Had tune-up and fuel system cleansed!