I don't know why your axis works on higher speeds and not on lower but I tried using gas springs on my machines for their constant spring rates and most of them failed rather quickly.
This is what I have found out about gas springs on the internet:
They are high friction devices and not meant for continuous operation. They are best for intermittent operations like hatches and doors where they can cool down between operations.
That still doesn't explain your problem. You have a rather large lead on your screw (20mm) and moderately sized steppers. Maybe the inertia of higher speeds helps to overcome the resistance of the ball nuts?
I would try a slower screw/ball nut or gear down if possible.