Ok I went to the track last night and changing the servo and adding a capacitor did not fix the problem.
I also tried installing a separate BEC, and the ESC kept resetting itself. I removed the switch, tried with the sensor cable removed, tried different lipos, re-soldered my connectors, tried various firmwares...
No joy.
Next step: put the ESC behind the wheel of my full-size car, and put the car in reverse. I'm done with Castle: I've had several car ESC's, and only one has been working correctly from the start. I've had a bunch of helicopter ESC's, they have been the subject of two recalls ( and they have a history of bursting into flame!)
Ordering a Tekin RS pro.