If it's not backlash, then you have a loose coupling or something similar. Notice that your "lumps" occur shortly after the machine changes direction, and are always on the same axis. Usually, that's backlash, but backlash would tend to be closer to the direction change. You have a "delayed" backlash, like it takes a few seconds for the backlash to occur. Like something mechanically is slipping a small amount, but takes a certain amount of force before it slips. It's definitely a mechanical issue.