The infamous battery light came on last night...

So i was bored last night and went for a cruise for about 1/2 hour, stopped to get a coffee (thats right, at night), got back in the car and started 'er up and the charge light stayed on. I started driving it a bit and it would flicker on and off depending on the rpm's, and after a while it would stay on below 2500rpm's. Also, the voltage gauge would sit lower than usual at idle, while my lights would dim quite a bit.
So first thing in the morning I start her up no problem, and the charge light does not come on. I take it to the local parts place and had my alternator and battery checked, both on the car and on a bench tester, and both showed no signs of any problems. Checked all the underhood connections and they looked clean, no damaged wires that I could see. The battery cable ends are in good shape as well. Could all this just be due to the underdrive pulleys not allowing a decent charge at idle when the system is under load (headlights, radio, fan, etc)? I don't see the voltmeter flicker at all during the day like it does at night.
Can anyone else think of anything else I can check or should I just accept it as it is? I'm going on a long trip in a month and I don't want to be stuck on the side of a highway with a dead battery. Sorry for making this so long....
BTW I have the stock pulley on the alternator and the idle is at around 700rpm

Thanks for the replies, I think I'm just going to see if the system charges properly for the next little while and go from there. I noticed the connector for the stator (little single grey connector) to the alternator has seen better days, I might consider replacing the whole wiring harness from the alternator to the power distribution box, I've seen someone post on here somewhere that sells the whole unit with fusible links built in for not too much money. The previous owner changed the alternator before I bought it last october, so its only about 7 months old. I think its a remanufactured one (dammit). Well, if it blows, its a good excuse to upgrade to one of those 200 amp jobbies And the battery appears to be 2 years old, so an Optima will replace it when it takes a dump.