1994 Nissan Sentra Car dies while driving/won't start now

My car was dying intermittently or wouldn't start sometimes. Was fine for about the last 2 weeks. Then we were told the alternator might be going out so we replaced it. After replaced, got about 20 miles down the freeway and it died out. Put the old alternator back in and then won't keep the battery charged. Got a new battery and the car made it home but then continued to die out the following day on my way to work about 5 miles from home. A friend said to replace the alternator, so I had it tested and they said alternator is good, so put the new one back in today. Now the car won't start at all, even with a jump. Please help!