94 GT 5.0 Charging problems

I have a 1994 Mustang GT 302 (5 speed) and it has an odd problem. Sometimes when I start it, especially when the car is cold, the battery gauge will only go up to about 10 volts instead of around 12-14 like it usually does. After a little bit of driving, usually it 'wakes up' and the battery will charge again, but other times it just refuses to charge the battery. I have replaced the alternator, most of the battery cables, the battery and the coil. The car will eventually die when I drive it if the battery isn't charging, the gauges will die, as well as everything else in the car before the engine does, and i can usually keep the car running for a short time unless i let the RPM's drop below idle (i don't know the numbers because the gauges are out i just know the sound/feeling of the normal idle in the car)

I've been working on this for about a week and a half now and just can't find the problem. Any help would be greatly appreciated.

Odd problem. I am not quite sure on this one. I had a problem like this with an 87 Crown Vic. You may want to have it looked at by a professional that can hook it up to a computer analysis system.