1999 Ford Mustang Charging Issues at or around 1500 RPM's

This car has basically been rebuilt front to back. Everything on it from the drivetrain, to the transmission, to the engine, to the suspension, has been replaced with top of the line performance pieces approximately 5000 miles ago by a reputable performance shop. The chrome Tuff Stuff alternator has the same 5,000 miles on it and the battery is a brand new Optima Red Top.

Here is my question. Under load or at approximately 1500-2000 RPM, the battery light comes on and if these RPM's are maintained or higher, the battery will eventually wear down and die. However, if I shift or keep the RPM's below this threshold, no battery light comes on an all appears fine. The alternator, even though basically new, was taken off and tested by a reputable alternator repair shop. It tests out fine, but they thought the belt might be slipping. So they put an OEM pulley on it to replace the chrome plated pulley. Put it back on the car, same thing happening. Took it back to them and told them to rebuild it, replace everything they could to include voltage regulator. Put it back on the car and replaced the old battery with the Optima. Still doing the same thing. There is no belt slippage whatsoever.

Of course, before I did the alternator thing and battery thing, I load tested both. Then and now I am seeing the same thing. 14.4 at idle, 12.2 with engine off, back down to 12.2 when at 1500-2000 rpm's and drops off from there under sustanined load. Checked what I could at far as grounds, but not sure where all are.

Help, this is very frustrating and I do not see any complaint like this on your website. I see complaints of the opposite. Meaning problems that are obious battery or alternator related symptoms.