I've found a solution. On the Jet Ranger I have installed there's a Governor toggle switch on the Collective. When I turned this off the engine settled. As this toggle isn't available on on all helicopters I set a button on my Joystick for the Governor toggle and I can once again fly the helicopters. The only caveat is that you must now control the engine speed using F1 and F2 or assigning to your joysticks.