Problem resolved. Wall wart failure.
Interesting case. Tiny "1 amp" 12v wall wart. I believe it to be the one delivered with the clock.
So after many years of operation, something failed and the voltage output drops under load. With no IN-18's installed, the output stayed at 12v. Adding even 1 tube load to the clock, the output drops to about 5v. With a full complement of tubes, the voltage output drops low enough to cause the logic to continually reboot.
Found a 12v 450ma older style supply with the right connector. Seems to work, but I am noticing some loss of GPS lock. I should take the clock apart again and measure system voltages, but if you know these clocks, you'd agree that dismantling and re-assembling them is no picnic.
Guess I'll be on the lookout for a 12v 1a rated supply.