I've had a recent problem with a couple of my liners, where the machine runs fine loaded, but once I hit the skin it starts to bog down and not put in a solid line, and almost makes a grinding sound. I've changed clip cords, played with the bend on my needles and made sure my front springs weren't worn out, and filed my contact screws of any build-up. I do use disposable tubes, but it happens with my steel ones too (but not as often). I feel this is obviously an error in my set up considering it happens with more than one machine. Any input would be greatly appreciated.