Phantoms are not designed to sit on the ground and idle. The motors rev up and down, as you put it, because of slight variations in the GPS signal. The Phantom is always trying to maintain it's position when the motors are running.
Leave your Phantom running for 20 seconds, without moving the controls and it will initiate RTH, if it works like mine does. 10 seconds later, it will try to climb to RTH altitude.
DO NOT LEAVE YOUR PHANTOM MOTORS RUNNING.
The Phantom is designed to fly once the motors are started.
The Phantom is designed to have the motors stopped upon landing.
Read it in the manual.
If you don't believe me, try it with props on and stand back, way back, especially after landing.
http://www.wynotweb.com/Bad_Phantom.html