voltage fluctations

The car was running great until I got stuck in some traffic. The temp gauge went up to 230 and I noticed my volt gauge dropped to 12 volts, even when I reved the engine the gauge stayed at 12 volts. The only thing running was my electric fan. After a bit of time she started charging again 14.5. I could drive awhile then it would drop to 12 again. It seems to do this at any time. I assume the VR is the problem..grounds are very good.
Thanks
AL