Hi Robert, hello Paul,
I also had a 40Mhz tuned Chess Challenger that failed sometimes with obviously bad moves, immediately losing a piece without any compensation. I downgraded the quartz to a 32Mhz one and it resumed playing perfectly (let alone the H8 bug that happens scarcely...). I think 40Mhz may work but you need a bit of luck with top of specs internal components, while 32Mhz is pretty safe.

Grüß,
Eric