Ah, there goes my theory, then.
i took off one time with one of my Engines at idle, (finger trouble as they were on a 3 position switch, ne for both, one for left and the 3rd position for the right engine ) AND the gyro (an igyro) on in the rudder heading hold mode.
Your F18 looked like my A10 behaved in yaw AND the way your model never seemed to quite stabilise in roll, despite the gyro, made me think.
Doesnt seem to apply to your situation. Sorry I couldn't help, and I hope you figure out what happened.