2006 JD 50D w/4k hours, Boom Drifts down, stops drift safety lever is pulled.

2006 John Deere 50D w/4k hours: When the engine/pump runs, the Boom cylinder drifts down, and the movement stops when the power is off or the safety lever disengages. The boom is warm to the touch but not abnormal; no external leaks from the left joystick or valve body exist. Any ideas/recommendations?