[BrianG]

You might want to check/replace the servo. I had a Hitec steering servo that would operate fine on the bench, but in the vehicle, it would sort of jam sometimes. Inside the servo case, there are several gears, and they ride on "axles". Well, it turns out that one of the axle holes had elongated and was causing a bad mesh internally, but only under a decent load. Since it was a MG servo, I just replaced the servo case and it worked fine thereafter.

Not saying this is exactly the case here, I'm just saying it could be the servo itself for whatever reason. If the servo is jamming and/or stalling, the high current flow could be dragging down the BEC voltage to the point where it is causing odd behavior in the ESC brains.