Bobbin thread tension is usually a top thread problem. I wonder that the upper tension disc may have gotten a groove in it from the thread cutting into it at fast speed. Just a guess. have you tired using this? It helps keep with the friction when thread goes through the disc. I use it even on my computerized machine. I've never had a problem and the repairman said it was the same lubricant that is embedded in the do not oil machines.
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