wherearemypistons?

The alternator seems to be cutting out on my 91 NA. All the warning lights used to light up above 7K, but now I'm getting the same condition at 4-5K. I checked my output with a meter, and the gauge is working correctly. (at idle I'm getting 13.7V, as I accelerate, the voltage is all over the place until I read about 10V above 4K) I changed all the belts today. (I thought the belt was slipping) Usually, alternators just stop putting out a charge -- I've never had one lose voltage as revs increase. Could it be the voltage regulator? How much for a rebuilt alternator? Thanks.

TODD