New Charging Issue

This February 2010 my alternator went out on my 2004 Trailblazer with 100,000 + miles. I replaced with a rebuilt, replaced the serpentine belt and replaced the original battery.
Just recently I have noticed my lights flickering and my volt gauge dropping to 12 volts and then back up to 14 volts while driving in town.
I went back to get another alternator and they tested it showing good with full load. Of course it didn't do it during the test. The guy recommended I check all connections. I did the battery, and alternator plug and output cable. All fuses are good.

This morning all seemed good for one hour then briefly it did it again.

By chance is there a relay in the circuit that may be causing this?
Any other thoughts as to the cause?

I thank you in advance,

Doug

how's the tension on the belt?

how's the tension on the belt?

Belt is tight. Watching it I don't see any flutter or vibration. Everything seems quiet. I listened with a hose near alternator and tension pulley.