On show day 2 the game developed lines in the graphics and then stopped booting. The spare game PCB was swapped in and the game ran for the remainder of the show with no further issues.
The game PCB wasn't booting on the bench. Testing with the Arduino ICT found that all the ROM, RAM and interrupt tests all passed. Checking with a replacement IC 117 (Z80 CPU) the game booted and played OK. A cross check with the original Z80 CPU confirmed it was bad and the game was fixed with a new Z80.
I'd had problems with this chassis drifting in and out of focus and the first attempted cure was a zip tie to hold the pot in place, however at the show the focus still drifted in and out.
The focus pot RV22, a RUWIDO 50M linear 15074004, measured open across all pins. A recycled one from a scrap chassis measured 36.3 + 35.7 MOhm that seemed reasonable and was used to replace it. Since the pot was held together with clips it could be opened to try to repair it, however the wiper was broken and not repairable.
Retesting the monitor chassis in the cabinet encountered no issues and the focus stayed sharp over several hours.