Random droput of ps_on during print (no replies)

I've selected POWER_SUPPLY 1 and enabled AUTO_POWER_CONTROL, taken 5v from RAMPS to SSR to ps_on, and it generally works fine... except partway through a print the PSU goes off for about 0.2s, which loses XY position.

When I connect the SSR -ve direct to ground instead of ps_on it all works perfectly - minus the auto control.

It's like the AUTO part briefly decides power isn't needed. The SSR apparently runs from 3v upwards so I don't think it's that.

Was wondering if anyone else has had the same problem.

I think I have v1.1.9 Marlin...

Thanks,
Phil