Chrysler LHS

Hello I have a 1994 Chrysler LHS and over the last few months we have had to change the battery, the starter, and now for some reason my car wont keep charge. The alternator tests that it is good, the battery tests that it is good but the car wont keep charge. I will be driving down the road and the battery light will come on and then with in minutes all my instrument panel lights come on then when I come to a complete stop my car dies and can't be restarted until I either get a jump or charge the battery. What could be possibly wrong with my car what should I check if my battery and alternator are still good? Any suggestions on how to tackle this matter?