True...especially if the RPMs are low when this happens. Granted, the Alt may be weak but for that sudden power fluctuation, I would look at the connections at the battery first.
Make sure the parts store gives your battery a free charge while you're there...
When our OEM alternators begin to die, they often slowly lose the ability to power the car and you'll see the volts at the instrument cluster drop as the battery is forced to pick up the job. It's normally not sudden with the gauge sweep and lights.
If it truly is the alt, you can eek out a bit more life from the Alt by smacking it on top with a hammer while running. You'll see the volts magically go back to where they are supposed to be, however after awhile, they'll drop again. Time for a replacement.
+1 on the Durango Alt. It costs less too.
2000 Laredo 4.7L Selec-Trac. 177K miles, Leather, 245 tires, tow package, crappy CD changer, full size spare, roof cross-rails & sunroof.
Changes & fixes: Rear Addco sway bar & greasble bushings, upgraded headlights, upgraded tail lights, new hood struts (replaced 2X), new rear hatchglass struts, new radiator (replaced 3X), new oil pressure sending unit (replaced 4X), 2 new speed sensors, 2 new door lock actuators, new ignition key cylinder, chrome front tow hooks, Mobil 1 Synthetic 5/30, Purolator Pure One oil filter, EBC slotted & dimpled rotors, EBC Greenstuff pads, new front shocks (replaced 3X), new rear shocks (replaced 2X), new IRO steering stabilizer, new timing chain cover, new water pump, thermostat, new thermostat housing, new high pressure power steering hose, Michelin-X radials, new sunroof assembly, new single-piece lug nuts, new exhaust manifold studs & gasket on driver's side, new Durango alternator. Jeepforum.com sticker, Visit cobaltboatsforum.com when you can!