Battery light at 3k RPM

I have a 94 Grand Prix 3.1 (197K mi.) With air. Recently although the car runs great, the battery light has been coming on at 3k rpm. I brought the battery in to be checked, checked o.K. Replaced serpentine belt, and brought the alternator in to be checked. I was told the alternator was o.K. Too. However, I did not tell them that the problem occurs at high rpm. Do they test the alternator at different speeds and does anyone have any ideas as to why I have this problem?