If 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.