Having had numerous stability problems, I decided to replace the fourth ESC unit which was different from the other three but which I believed was essentially the same as the others due to it being a clone. I should have been alerted by the fact that the Turnigy programmer would not recognise it. I changed it for one that was the same as the other three and hey presto...stability and enjoyment returns. This is a composite video of my last two flights, the second part is more interesting. Sorry about the shaky video but I was flying at the time. I also set the IMU and GPS offsets to zero again which has also aided stability. Does anyone know why my quad is reluctant tor yaw anti-clockwise but easy to yaw clockwise? Comments gratefully received.