amp guage

Hi all, I'm having a problem with my amp guage or electrical system on my 94 Sierra, 5.7L. When stopped at a red light /stop sign with my headlights, defroster and radio on, my amp guage falls almost to the red zone. A clicking sound can be heard just before it happens. I have replaced the battery and alternator (110amp). Are there any relays/fuses that might be going bad that would cause this?
What rpm should my engine be running at while stopped with trans in gear? My tach reads about 400rpm. Out of gear at idle it reads about 800rpm.
Thanks