Z axis occasionally gets jammed, inexplicably gets unjammed
Once in a while, when I raising or lowering the Z axis, it'll appear that the Z axis will get stuck. When the bed is travelling it'll suddenly freeze and I'll hear a loud hammering sound. This isn't a constant occurrence but has happened enough to be a pain and has caused a few prints to fail to start because the bed didn't fully raise at the beginning of the print.
I've lifted the BoXZY onto its side to check if the motor is obstructed and have confirmed that there's nothing jamming the motor or the drive belt. When the axis is jammed the motor will twitch and jerk when it's stuck, which I assume is the source of the hammering sound. The Z axis looks clean and lightly oiled, so there's no obvious reason why it is binding up.
Speed doesn't seem to be a huge factor in when sticking occurs; I've run the Z axis up and down at 4800 mm/s and at 50 mm/s and it's stuck at both speeds. However, direction does seem to be more of a factor - I believe that I've seen the Z axis stick more frequently when travelling up than travelling down.
To make things more interesting, when this sticking does occur, after a while it'll magically become unstuck and will behave normally. I'll try raising and lowering the Z axis while it's stuck, and at some point it'll become unbound and behave totally normally.
Is this a known issue, and is there anything I can do to diagnose what's going on?
Can I try lubricating the Z axis, and if so what grease should I use?
Edit: I just had another print fail at the start; I sliced it with Cura engine and after the extruder was zeroed and heated up, the X axis was suddenly raised and skipped a bit. When the print started the filament didn't make contact with the bed and "oozed" down to the bed haphazardly. I restarted the print and it didn't occur. The related G code is at https://goo.gl/xplJGb.
Is this a good question?