Y axis suddenly moves twice as far

The machine has been working fine for quite a while now but today between two jobs the Y axis moves double distance.
I switched the machine off/on and rebooted the computer. Home all axes and move to X100, Y100 but Y axis actually moves 200mm (X is correct). I haven't been anywhere near any config file and as far as I can see X and Y are configured the same except for home amd limit positions.

What is going on?

LinuxCNC 2.5.0
Config files attached.

I would suspect the step drive microstep settings have changed (especially if they are set with dip switches)

LinuxCNC drives the steppers using the parallel port. The machine just takes these signals and drives the steppers. There is no 'brain' in there and no dip switches.

There must be a step drive between the parallel port and the step motors

Most step drives have ways to set the microstep ratio
changing the microstep ratio inadvertently would cause the type of symptom you have

PCW wrote:

There must be a step drive between the parallel port and the step motors

Most step drives have ways to set the microstep ratio
changing the microstep ratio inadvertently would cause the type of symptom you have

Today I have had the box apart and boards out. The drivers use a A3977SEDT chip has two pins (MS1, MS2) to set the ratio. Both of these pins are connected to +5V on all three boards - no switches/links. This selects eighth step.

The problem is intermittent but doesn't respond to percussive maintenance (!)

To get double speed MS1 must be low. That could be a duff joint on the PCB or it could be inside the chip. I put a solder blob between the two pins so they must both be at the same potential. This made no difference, so the problem must be inside the chip. These chips are no longer manufactured but are available on eBay. £24 - ouch! and won't be here until 24 May but it gives me a chance to practice soldering PLC44 devices!!! Wish me luck.

You might try a bit of freeze mist on suspected parts to try and locate the intermittent

The new chip arrived today and I have fitted it and it all seems to work. As the problem was intermittent it is possible it is trying to lure me into a false sense of security...

When I posted the original question I was expecting it to be a software problem but the replies here pointed me at the hardware and it turned out to be a stepper driver chip which had developed an unexpected 'maybe' gate so thanks.