Help! Car won't start

Please help me. My sister has a 1999 ford escort. I replaced the alternator and the battery is brand new. When the alternator is in the car it is not giving out as much power, so the battery is not being charged. The car will run for a while. It seems like the alternator is bad, but it is brand new, and when I take it out and take it to the shop to test it, the alternator tests perfectly. What the hell? I have spent the last 4 hot, humid nights working on this here in Houston, Tx. Please help me so I can get out of the heat. Thanks.

well that sucks...not to long ago the same thing was happening to my camaro my dad took the stock alternator that was in the car and it was just fine so he said...forget it and bough a brand new one...and that works just fine...sometimes spending a couple extra bucks for a better quality one helps out ...good luck

yeah...i just put in a new alternator. I thought that would fix it, but still same problem.