2000 Mercury Cougar Alternator

I've had 3 alternators put on this car and installed 2 batteries in less than a year and here is the problem I keep having.
The car loses power while driving. The instruments start going crazy, it starts spitting and sputtering and then it dies. When it dies I mean you can't even roll the windows down or blow the horn or even take it out of Park until you jump start it. This gives it just enough power to start the car but when you take the jumper cables off it dies again immediately. I've had 2 separate electrical specialty shops look at it and they can't find anything wrong. They switch the alternator and it works perfect for a few day then repeats itself.