Mine is doing the same thing! It's 1.5 yrs old. and I LOVE it--except for this! When I adjust it back to "3" the dial continues to move in the same direction I adjusted it. IE I tighten it to "3" then it continues to tighten, so I loosen it back to "3" and it continues to loosen.
I'm driving 45 minutes to my nearest "authorized" repairman, and he can't find anything wrong with it. He's also lecturing me about only using the exact same wright and brand of thread off the spool and on the bobbin.
Could the vibrations from the machine cause the knob to move? It was suggest to cut up a yoga mat and put it under the machine to absorb the vibrations--if that's the problem.
I'm tempted to take it somewhere else but don't want to void the warranty.