alternator not working above 4500 RPM
 

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