Without knowing the exact specs of the servo i'm speculating but my theory is that the servo is fading out due to heat and load from pressure on the rudder doing the maneuver over an extended period of time.

You said the plane maintained a good knife edge for some distance before it happened the first time, so perhaps the pressure on the servo and strain caused it to heat up and fade.

On the 2nd and 3rd attempts it probably happened earlier in the maneuver due to the latent heat remaining from the first instance.

I have a Venus II and use a Futaba 9151 Digital F3A rudder servo with 132 oz-in. torque and use a regulated 5.5V power source. My Venus will knife edge the length of our 1/4 mile strip without a problem at all.