The 206, 306, and 319 machines were made to use 206X13 needles. If the machine hasn't been modified for 15X1
needles those timing tolerances will be off and you will have all kinds of problems. You can also damage the bobbin
case if using 15X1 needles. Even if they were modified I don't think they run as good as they do with 206X13
needles. You can still get 206X 13 needles on line. I would start there and see how things quiet down. If your
bobbin case has been damaged you should replace it. There were two kinds of bobbin case so make sure you
get the one for your machine. The operators manual will tell you that you must use the 206X13 needles.
If someone has tried to modify the timing or maybe put in the wrong bobbin case you will have fits with the machine.
Also the 206, 306, and 319 use their own special bobbin that you have to buy now online.
These are one of the few singer machines that took special needles, bobbins, and bobbin cases.