Its annoying but most hobby servo datasheets say nothing about the peak stall current, which is the one
important figure you need to know to power them. If the supply cannot provide enough current for the peaks
the servo will typically crash/reset/go haywire.
You should never think of powering servos and motors from the logic 5V supply of the Arduino, that's totally
underpowered and risks damaging the Arduino from stray voltage spikes. Motor supplies and logic supplies
need to be separate for a well behaved system.