Assuming you've checked common issues like incoming 3-ph, controller & robot connections, robot-to-controller cables, robot wire harness ...
Also, assuming this is a 2400/16 or /10. Can you jog A5 only or A6 only in joint? Use the joystick lock if needed. Can you further isolate it a single axis? Is there any movement at all on the upper axes when trying to jog them?
Not to say this is the problem but I've seen similar symptoms with an open motor phase or a motor phase arcing/leaking to ground but not fully shorted.