By engaging and dis-engaging at will, do you mean even if you were driving down the road it will engage on its own, or once engaged, it will go in and out? If the later, it may still be electric.
If a dealer did the rebuild, have them bring out their flow meter and pressure tester. Having the pressures and flow read makes so much easier the diagnosing of that clutch.
But it is entirely possible that assembly was rushed or that a seal pressure check was not done when rebulding.