Alternator problem 01 GTP,3.8

Is there anyway to adjust the output voltage on the alternator? I hooked a volt meter to the output terminal leaving the car idle.The voltage read 14 + but after a time the voltage began to drop. When it got down under 12 volts the engine started to missfire. Reving the engine did not change the voltage. Turning the car off and restarting it ,the votage went back up to 14+ and the missfire was gone. It does not do it all the time. I suspect the low voltage is causing the engine missfire.
To those that helped diagnose other problems on this car (my daughters) yes it is the same car. The alternator may be the culprit behind some of the other problems.

You can have the battery and charging system checked for free on the car at any Advance or Auto-Zone if you have one in your area. It sounds like the recifier bridge/diode assembly inside the alternator is failing when it gets warmed up, but it could also be a battery with an intermittent short or dirty/corroded cables (they create excess resistance that compounds as the engine compartment gets warmer).

Thanks for the reply. It is a new battery and the cables checked and cleaned. I had the alternator checked at one of those stores and it tested fine. The car does not have to be warmed up ,it will do it cold. As i menthoned it only does this on occasion.I'm inclined to beleive it is an alternator internal problem as you suggested. In the old days the voltage regulator was mounted on the firewall and could be adjusted. Of course the old generators could not keep up with the requirments of the new engines and accessory's of today. Going to get a new alternator and post the results at a later date. Thanks again

Well back to the drawing board. When started with new alternator it started missfiring and the voltage read 12.2 volts. Turned key off and restarted ,ran fine voltage over 14 .It hasent done it since but I don't think it was the alternator. Whatever it is. it does not do it all the time. I keep hearing about bad ignition switches on these models . I tried to jiggle it while it was running but could not cause the problem. Everthing you can think of has been changed out ,still no luck. Where else can I look??????