I have a 91 z28 with the 305 tpi. When I start the car, the voltmeter on the dash starts of at the right side of the meter like it should. After about 15-20 mins worth of driving it has fell to about half way on the meter, which is about 12 I think, which doesnt seem too odd. The strange thing is that when I come to a stop it has a slight hesitation every 5-6 seconds where the tach will fall slightly then pick back up. Also when I increase the load on the car by turning on the defogger, lights, etc., it makes the voltmeter gauge drop drastically into the far left side of the meter and the car starts running really rough like it cant handle the loads and run smoothly at the same time. I have had the battery and alternator tested and they all check out fine. I had an amplifier that is about 700 watts and two 15 inch subs hooked up to it, which I thougt was the culprit. They were pulling a little too much I think, but I unhooked the power source for the amp from the battery, and it still is doing the same thing, it just takes it longer for it to start giving problems with the amp and speakers unplugged. Any help at all would be greatly appreciatied.

 

I still think it's your alt.
Hook up some temporary wiring to a voltmeter:
positive to your alternator post
negative to your battery

Run the wiring inside the car to a voltmeter and go for a drive. Try to duplicate what it's doing and check your reading.

You should be in the range of 13.8 - 14.3v

 

thank you, I will definitely check it out