I have a 2001 Pontiac Trans AM with 5.7L WS6 pkg. and auto tans. with 22,435 miles

On cold start the in dash voltage meter reads at about 13 volts where it should be, as the car reaches operating temp, the voltage gauge starts to drop and when the car is idling, it will sometimes dip down into the orange/caution area and shudder like it wants to stall. When you start driving again the voltage comes back up slightly, but not back to "normal" - Just replaced the alternator, battery, idler tension pulley and serpentine belt - stall having same issue - Voltage test show full charge on the battery and no issues with alternator - any ideas?

Thanks!