@KenW
You brilliant bastards, you've done it again. This was it. Expansion board was slightly behind the mainboard. Thanks for all the great suggestions gang.
For future generations:
M115 to check mainboard firmware version.
M997 to update firmware on mainboard
M115 B1 to check firmware version on expansion board at CAN address 1.
M997 B1 to update the expansion board.
