If you haven't already, try reseating both ends of the ribbon cable between the CPU board and the driver board.

I wouldn't recommend buying one yet, but if you can borrow another CPU board, try that. If that's not an option, I would try removing the CPU board and reseating the ASIC chip. You need a chip puller for that. Once you get the chip out, look for bent pins. You can usually straighten bent pins with a small blade. Bent pins or not, reinstall the chip and see where you're at. One pin being off on the ASIC can cause some bizarre errors. In my experience, that error is usually isolated to one coil/ device.

STTNG didn't get any changes to the cannons in software updates, so you can move whatever game ROM you currently have to the test board and the cannon should work properly.