I'd try changing the needled again. I just had this happen. I put in a new needle, sewed for about 2 minutes, still had bobbin thread problems. So I put in a second new needle and all was fine. There must have been a defect in the first brand new needle I tried. I threw it out.
Perhaps it is the design...are they from the same designer or collection? I might suggest trying the design that you are having a problem with by itself...as a test...rethread the machine, put in a new bobbin and if it still sews out poorly but you other designs are fine...I would say it is the design...If it is a purchased design, try contacting the designer and advising you are having a problem...perhaps you are not the only one...good luck!
My thoughts too, I purchased 3 different designs for a custom order and the lettering is the only file I am having problems with. they are all different digitizers. I was stitching my test and all this happened it is done testing but plan on testing the lettering again.
If you still are having a problem with the lettering, perhaps it is something I can digitize for you...if you need help, PM me and I will try to help if I can...
Are you using a pre wound bobin that is less than half full. Sometimes I get problems with one of my machines with bobins that are getting to empty. I would also try re inserting the bobbin
I recently put in a new prewound bobbin and had nothing but problems, till I decided to run my finger along the edge of the new bobbin. It had a tiny nick and that what caused all the problem like constantly breaking the thread. Just a thought.