Normally if it's the regulator, the voltage will go through the roof, popping fuses, boiling the battery, etc - sinceit regulates the voltage down, not up.
I'd say either belt slip, bad connections, or a bad alternator.
It seems like a lot of people have similar problems as yours when they have a bad ground somewhere in the electrical system. You might be able to trace your grounds (under the hood) from the negative on the battery to where ever they go and check for corrosion or bad connections. Hope this helps. Maybe some of the others on here can help on where to look specifically for problems. Good Luck.