I have the very same "top thread showing on bottom" with my Janome. Mine is a very inexpensive Janome 415 model that I bought to carry around. The tension on the stitches has been an issue forever. had the darn thing in the dealer numerous times. Just got it back, did about 50 inches x12 inches free motion on a quilt and the bottom thread was already getting loose. I have changed threads, slowed down, used different stitch lengths, and no better results. I'LL NEVER BUY ANOTHER JANOME, nor will I ever recommend one either. My old 1967 Singer is a consistent machine. BUT the feed dogs don't drop. I'm working on figuring out a way to do my FMQ on that machine. The tension is good even if I don't get it serviced but every 10 yrs.. literally.... Keep taking your machine in with examples and see if they can fix it. OR replace the machine as a lemon....