Ok. I found and corrected a problem, but I don't know if it was the problem.
The voltage from the main panel checked out ok. 243 V total, 60 Hz, 50% duty cycle. 121.5 V, 60 Hz, 50% duty cycle to each leg. The voltage leaving the breaker was 243 V, 60 Hz when on, and 700 mV AC with 0 Hz...