Crunchy X-axis on Duet Wifi

The Duet forum is not accepting any more registrations, so I can't ask there but I'm having exactly this issue on the X-axis
[www.duet3d.com]

It's weirdly intermittent, as occasionally it drives correctly, as the previous poster says. Although I find it doesn't last and will change to this weird crunching after some time.

I've also Changed the loom and changed the NEMA17 motor it is hooked up to and tested against my 2 year old (always working) Ormerod 2's X-axis (with Duet 0.65 running firmware 1.19) as well as the Y-axis on this one - same symptoms.

Video of symptoms - [photos.app.goo.gl]
this video is immediately afterwards, after swapping the X and Y axis cables on the duet and nothing else - [photos.app.goo.gl]

I was running Firmaware 1.19.2 and just in case I upgraded to Firmware Version: 1.20RC4 (2017-12-20) with no change to the behaviour.

config.g is

; Configuration file for Duet WiFi (firmware version 1.17 to 1.19)
; executed by the firmware on start-up
;
; generated by RepRapFirmware Configuration Tool on Wed Dec 20 2017 22:03:57 GMT+0000 (GMT Standard Time)

; General preferences
M111 S0                            ; Debugging off
G21                                ; Work in millimetres
G90                                ; Send absolute coordinates...
M83                                ; ...but relative extruder moves
M555 P2                            ; Set firmware compatibility to look like Marlin
M208 X0 Y0 Z0 S1                   ; Set axis minima
M208 X240 Y240 Z200 S0             ; Set axis maxima

; Endstops
M574 X0 Z0 S0                      ; Set active low endstops
M574 Y1 S1                         ; Set active high endstops
M558 P1 X1 Y0 Z1 H5 F120 T6000     ; Set Z probe type to unmodulated, the axes for which it is used and the dive height + speeds
G31 P600 X0 Y0 Z2.8                ; Set Z probe trigger value, offset and trigger height
M557 X15:215 Y15:195 S20           ; Define mesh grid

; Drives
M569 P0 S0                         ; Drive 0 goes backwards
M569 P1 S1                         ; Drive 1 goes forwards
M569 P2 S1                         ; Drive 2 goes forwards
M569 P3 S1                         ; Drive 3 goes forwards
M569 P4 S1                         ; Drive 4 goes forwards
M350 X16 Y16 Z16 E16:16 I0         ; Configure microstepping without interpolation
M92 X87.489 Y87.489 Z4000 E420:420 ; Set steps per mm
M566 X300 Y900 Z12 E120:120        ; Set maximum instantaneous speed changes (mm/min)
M203 X6000 Y6000 Z180 E1200:1200   ; Set maximum speeds (mm/min)
M201 X500 Y20 Z250 E250:250        ; Set accelerations (mm/s^2)
M906 X800 Y1000 Z800 E800:800 I30  ; Set motor currents (mA) and motor idle factor in per cent
M84 S30                            ; Set idle timeout

; Heaters
M305 P0 T10000 B3988 C0 R4700      ; Set thermistor + ADC parameters for heater 0
M143 H0 S120                       ; Set temperature limit for heater 0 to 120C
M305 P1 T100000 B4138 C0 R4700     ; Set thermistor + ADC parameters for heater 1
M143 H1 S260                       ; Set temperature limit for heater 1 to 260C
M305 P2 T100000 B4138 C0 R4700     ; Set thermistor + ADC parameters for heater 2
M143 H2 S260                       ; Set temperature limit for heater 2 to 260C

; Tools
M563 P0 D0 H1                      ; Define tool 0
G10 P0 X0 Y0 Z0                    ; Set tool 0 axis offsets
G10 P0 R0 S0                       ; Set initial tool 0 active and standby temperatures to 0C
M563 P1 D1 H2                      ; Define tool 1
G10 P1 X10 Y0 Z0                   ; Set tool 1 axis offsets
G10 P1 R0 S0                       ; Set initial tool 1 active and standby temperatures to 0C

; Network
M550 PMy RepRapPro Ormerod 2       ; Set machine name
M552 S1                            ; Enable network
;*** Access point is configured manually via M587 by the user
M586 P0 S1                         ; Enable HTTP
M586 P1 S1                         ; Enable FTP
M586 P2 S0                         ; Disable Telnet

; Fans
M106 P0 S0 I0 F500 H-1             ; Set fan 0 value, PWM signal inversion and frequency. Thermostatic control is turned off
M106 P1 S1 I0 F500 H1:2 T45        ; Set fan 1 value, PWM signal inversion and frequency. Thermostatic control is turned on
M106 P2 S1 I0 F500 H1:2 T45        ; Set fan 2 value, PWM signal inversion and frequency. Thermostatic control is turned on

; Custom settings are not configured

Setting maxmium speed , M203 X to 2000 (i.e. M203 X2000 Y6000 Z180 E1200:1200 ) makes it a little bit better, but it's still crunching as it moves...

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Today I was demonstrating to a friend and he pointed out that to him, it looked like it might be heat related, as it works from a "cold" state (not been switched on recently) but doesn't work after it's been on a while.

We then did further tests by pointing a big household fan at the board when switching it on from cold.Many (unscientific) repeats seem to show that it works a lot longer when a fan is pointing at it, as well as failing after a couple of minutes when the fan is removed.

I guess it could be a dodgy driver. Can you temporarily use the E1 motor output instead to check that?

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].