Hello Frank,

It's unlikely that you will need such high current for your steppers, even with a load on the mount. Try using about 40% of max current first, set your IRUN to 400~500.

Some stepper motors will resonant without a load high currents or step rates. It's possible that you're overheating the TMC2130 drivers with that much current and the overheat/overload protection in the driver is shutting off the current.

The last possibility is an intermittent connection to the stepper driver or the stepper motor, of course check carefully with a light and magnifying glass.