Check the steering linkage is running smooth and free...and can turn fully each way without restrictions while the servo is powered down.
I wonder if some part of the linkage that moves near the bulkhead might be catching...and popping past an interference once the servo gets the full load on it???
Also, is the servo spaced correctly away from the steering linkage on it's mounts? If it's too close then the end of the steering arm can rub on the front of the servo, and might behave like you've described?
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