Voltmeter going haywire, could it be faulty?

I will be driving along and my (stock) voltmeter will just all of the sudden jump down to like 10. As a result the battery idiot light comes on. Tnen it will jump back up to ormal after a minute or two. The battery shows no signs otherwise of going. Could this just be a faulty volt gauge? Does this happen at all ever? I have a 92' with 120,000 mi. Thanks

oh yeah, and I have a new (rebuilt) alt in it, so I dont think its that

Yeah, it is the stock amp alt. About a year back I had a small fire.....the protective coating of the wires going to the alternator rubbed off and touched eachother. I had that fixed......they spliced in new wires and put in a new rebuilt alternator. Then a couple of months ago, that piece of junk rebuilt started sounding like it was full of rocks so I had that replaced. Now I have this problem. I swear, my Mustang has been nothing but a great, reliable car for me now for 6 years, but it just seems like I have a damn alternator problem like every year!