Been having the same problem with a 2.4ohm head on a mini vivi nova. I've been using it with two different Twists and a Spinner, all initially set at 4.2 volts. With each battery, if I turn it down to 3.7 volts the problem goes away. I was thinking the resistance on this 2.4ohm head must have decreased by a lot, but I don't have an ohms meter so can't test that theory. Do you have a way to test the resistance of your head?