LAOS Laser

Hi,

I have a LAOS-driven, chinese K40 lasercutter. Lately it has been decidedly unstable: for a while it crashed only at the end of jobs -- it'd finish the job, return to home, then crash -- but more recently it has been crashing during jobs, particularly ones with lots of small, intricate movements.

Crashes take the form of the X and Y axes suddenly stop moving with laser off. The I2C screen stays unchanged, and still beeps in response to button presses, but does not otherwise respond. If the mBed is connected to a computer by USB at the time, the terminal stays silent (as it always does while cutting), and the mBed can still be mounted as an external drive.

The LAOS board has a 5V (logic) and 24V (motors) supply from a 5A transformer separate from the laser supply. The ground of this is connected to the ground of the laser power supply, confirmed using multimeter. So it doesn't seem likely to be power supply issues.
Connections between the I2C screen and the LAOS board seem solid, and will take a fair amount of poking and prodding without inducing a crash, when the machine is on but not cutting. So a dodgy connection there doesn't seem to be the problem.
While there are files (and sections in files) that are much more prone to crashing, crashes don't reliably happen in exactly the same place when the file is re-uploaded.
Several rounds of [upload file] -> [start cutting until crash] -> [save file from SD card before power cycling] confirmed that the file received and written to the SD card is identical every time, so it's not a problem with the network or writing to the SD card.
Possibly related to the above: during startup, the LAOS screen always says "SD NOT READY!", but as it has always done this, including when it was working perfectly, I assume this is just an oddity of the firmware?

Crashes are much more likely to happen when cutting intricate details with lots of small movements: 2mm diameter circles crash extremely reliably (only manages 2-4 circles) or the hexagons in the attached file (8-46, usually about 30). Slowing the cut down dramatically doesn't make a difference. I'm pretty sure that the intricate detail itself causes crashes (certainly with the tiny circles, which reliably crashed within a few seconds of starting), but presumably just the fact that it makes for longer cutting time is a factor too. I won't pretend that I've done a proper analysis of that.

I've attached the config of our lasercutter, the visicut file, and the simplecode copied from the LAOS board's SD card after a crash. This was using the v5 board with the 2015-03-18 firmware from LaosLaser/Firmware on github.

Questions:
Any idea what troubleshooting steps we should try next?
Is there any more information from me that would help diagnosis?

cheers,
bugs (Sheffield, UK)