Weird Electrical Problems

I have a '79 camaro. A couple of days ago I noticed that my headlights were getting dim. When I checked my voltage gauge, it was almost in the red. I thought that my alternator had died, so I replaced it. However, this did not fix the problem. I replaced the battery, but the problem persist. The weird thing is that as long as the car is in park, the voltage gauge will read around a 12, but as soon as I put it into drive it drops to the red (with or without the headlights on). What's up? Does anyone know?

I think you may have just bought a brand new bad alternator..... It happens

did you replace the volt regulator also?

How's the belt tension? Sounds liks the alternator isn't being turned at the correct speed. No other 'odd' noises from the other pulleys?

That idea crossed my mind this evening. I don't have any belt noises, but that doesn't necessarily mean anything. I'll check it out. Thanks.