retrolefty,
This sounds like a hardware issue.
@OP,
How about drawing a schematic by hand with pen and paper and take a photo with your cell phone. Get a multimeter and measure the voltage on your external power with one servo and then add the rest of the servos one at a time measuring the external supply voltage after each addition. Make sure you have a common GND. Post the schematic photo and the measurements. A photo of your circuit might be requested in the future if the situation doesn't improve.