Dodge Repair: Charging system, junk yard parts, dodge intrepid

Question
1993 Dodge Intrepid 3.3 Litre engine. Problem I'm having is, when the car is running, The alternator doesn't seem to be charging. I have always checked with a Multimeter across the battery, not running usually around 12.5+V then running should be 13+V. I get the opposite on this car, it's lower when running, I think it's just running on the battery, but I have noticed a couple of times while the meter is hooked up, that it starts to gharge 13 - 14V on meter but only lasts, less than a minute. I have gotten 3 different alternators from junk yard, all seem to work the same. Leading me to think, there is something else that I'm not seeing as part of the charging system.  

Answer
Mark,
Have you solved your charging problem yet?
Are you aware that the alternator is regulated by the pcm,
and that it is not uncommon for the pcm to lose its
ability to regulate the alternator.
I would check ALL grounds.  The pcm controls the field
on the ground side, not the pos. So if there is a bad ground, the pcm cannot control the alternator.

Also...I have NEVER trusted junk yard parts...