other thing you can do is to bend the back spring up a little or the back spring may have a crack in it also is every thing in a line , a pic will help its hard to say but give it a go, ive found if the machine run well but needs a jump start ,you need more power ,but in saying that if you have say7v 0r 9v and its doing that check the arm'bar is over the coils right , if so start bending your back spring up and tune it ive found this works when doing shaders that i want to run at low volts but still have lots of wackIf your machine stops and the bar needs a jump start, then i would have a stab at the contact screw being worn, or a cheap one. Try getting a silver one, or a better quality one.
Another option maybe to alter the gap between the spring and the contact screw. This will affect your machine tho, and may need a bit more power to compensate.
Im no pro myself, but these seem the most likely options. I find that stripping the machine down to nothing and re building it teaches me quite a lot about little problems like these.