I have a 1995 Mitsubishi Montero, new battery and new alternator. Car loses charge while driving, can actually see the battery gauge going from 16 to 12 in ten minuets then gets to 8 and radio goes off and lights dim till it gets to 0 and dies...if I get a jump for 5 min. it will drive another 5-10 minuets before it does the same thing. If I charge it with a charger for 3 hours it will run for 20 minuets before starting to lose battery power and get's down to 12 by the time I get home, then it sits for about 3 hours and charges back up on it's own.

You need to check the charging system. Take a volt OHM meter and go to the positive and then the negative posts. It will tell you how many volts it is putting out. If it is not charging at least 13.9 volts, then you bought a faulty alternator. If the alternator is good then check the belts to make sure they are tight enough. When you push on the belt at about 3 inches from the alternator it should not give more than 1/4 to 1/2 inch. If it does move more, then tighten the belts. If it is a V belt, just tighten the alternator. If a serpentine belt, check the belt tensioner and belt routing to make sure it is on right. The diagram should be under the hood.