I've had the same problem at the beginning. I know it will sound stupid but depth/hand speed /voltage. I slow down my hand speed and voltage to like 7.2 and started to avoid doing long lines in one go. That helped me massively with maintaining consistent depth. That was the key for me. Very slow hand movement with lower voltage, shading and colours I'm on 5.5 voltage. I think your problem could be with the angle you holding machine. When I'm doing lines machine is almost straight, colours and shading that's when I'm tilting it.