These older cameras just go buggy some times. I processed 8 rolls of 120 tonight which I shot last weekend on my trusty RZ (never had a problem) and about half the frames were wildly underexposed, just the opposite of what you experienced. There's almost nothing there. I'm seriously pissed right now, as I spent a total of four hours in the car to shoot these 8 rolls!
Stick with the camera that is working, and keep your eye on it, as it's liable to STOP working. :-)