That sounds like a bad alternator not putting out enough juice. Eventually you will have to buy both alternator and battery if you run the battery down too far. I can sympathize though. I'm on my third alternator, and still have charging issues.

If anyone can help me out, my symptoms are 13 volts constant when the headlights are on, 13 with them off, but <12 when off and I rev the engine or put load on the truck (IE, go up hill, passing breeze, etc..). Once it goes below 12, it stays there until I turn the lights on, and then let off the accellerator.

I've resigned myself to running with my lights on all the time. Mechanic says it's just fine (new factory alternator, new battery). Oh yea.. 1997 Isuzu Rodeo, 60K miles.