Stereo & Electronics - HO Alternator - problem - lights dim at 4000rpm

I had installed a high output alternator - the Alterstart 275 amps about 8 months ago - anyways I started to notice (last 3 months) that when I rev to around 4000rpm or higher (when driving) - my dash lights and head lights start to dim slightly and when I left off the gas - everything brightens back up. It also does this when I'm in netural. Is this normal??? I switched the alternator belt to the gatorback one when I installed the new alternator. Any ideals? Do you think the belt is slipping?

Did you check these already?

Alternator pulley tight?
Is the belt routed correctly? at the alternator?
Is the belt tensioner keeping the belt tight?

make sure your battery connection is clean and tight, check the power wire going to the alternator as well.

it honestly sounds like an issue with the voltage regulator. its doing the opposite of what usually happens, alternators usually increase output at higher RPMs.

As for the tensioner and the belt - I've tested by hand - they seem tight and very little play in the belt. The belt is on the grooves.

How should I test the tensioner?

if there was somthing wrong with the belt or tension, u will hear it skweak eaven at lot rpm.

im thinking its a bad voltage regulator, which im 99% sure is internal in the alternator. take it back and get a new alternator (if it has a warranty).