I have a 1996 Mazda 626 that seems to be having an electrical problem. I just put in a new battery, and have tested the voltages while the car is runing and the battery is loaded. I'm not seeing a signigicant change in the voltage, it's running between 13 and 15 volts. Unfortunately after the car has travelled for various times the electrical systems seem to shut down. I'm kinda stumped.. alternator.. voltage regulator ???

I don't quite understand the symptoms as you have posted them.

Please re-post and give more detailed info.

Well Let me try to re-state this.. The car runs well.. It idles and runs well. But there seems to be a charging problem with the electrical system. The car has a brand new battery. The car starts without a problem. After it has been running (driving) for some time, and this varies.. the car experieces what appears to be a loss of electrical power. The power to the radio, lights, and dashboard lights begin to fade and flicker. We can park the car, and put a battery charger on it for a bit and the car will be fine for a bit. The power drain has repeated itself on many occasions with driving distances from 4 to 30 miles..

I agree with you, it does indeed sound like it could be an alternator/regulator problem (they're integral), and also temperature related).
The voltage between a fully charged battery and a discharged one is very small, but monitoring the voltage while the car is running can tell you when or if the system stops charging. You can do this by hooking your meter to a cigarette lighter plug and utililizing the lighter socket for this purpose. The readings you're presently getting are in the normal range when everything is turned on. Some auto parts houses have facilities to check the charging circuit without removing the alternator assembly from the car. Their instruments do a different type of check that's better able to spot problems than simply monitoring the voltage, so it's something to consider. There are some other things that on occasion can cause mysterious symptoms, but other than checking for a slipping belt, loose connections and such as that, there isn't much you can do without other equipment and following a troubleshooting procedure.

It's the distributer. Check to be sure you still have the rubber seal between the distributer and the cap. You are getting condensation in the distributer during warmup - shorts out the electronics. Warm low humity days are fine but cool humid days cause the condensation and subsequent short in the dist. I replaced mine twice before I found that the seal was missing. Replaced seal and had no more problems.