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Have you checked the valve? With a pressure gauge on the valve, turn the power off and close the shut off valve to the jack. Are you losing pressure? When the unit is trying to warm the oil, could the car be drifting down? If you feather the manual lowering will it fail and give you the same faults? What about re-leveling? Is it always at the bottom floor?

Johnjays first post was pointing in this direction. I seem to remember this type of trouble. Just wish my memories were clearer.