i've had the same thing happen on two other brands of machine, also. i guess i'll have it happen, sooner or later, on my janome but i've only had it since November ... knock on wood.
unfortunately, my point is that i don't think it was the machines. in each case it turned out to be some teeniny little detail of somethingorother i'd done not exactly right. (do NOT ask how many times it turned out that i had put the bobbin in backwards. :shock: :oops: :roll: :lol: )
the critical clue may not be what steps or combination of steps you've tried to fix it. rather, what condition or combination of conditions existed when it started.
there is - for me - one unanswered question in your specific predicament. did the problem start while you were sewing? or did it start after you'd loaded something new ... top thread? bobbin? needle?
if it started while you were sewing ... do you remember how much thread was left in the bobbin? on the top spool? how long you had been using that needle?
if it started when you began sewing after changing something, what was that "something"?
take out the top thread. take out the bobbin. take out the needle. put the feed dogs back up. put any regular (or the walking foot) back on. reset the tension to the automatic setting.
walk away and take a break. stay away from the machine for at least an hour. do somethin - anything - that will distract your attention so you can come back to it with a clear, refreshed mind.
start again with scraps, a needle you know is new and of the same size and type you were using successfully, a bobbin of the same type and size you were using when all was well - full of whatever you had in the bobbin when it was working right, and the top thread you were using when things were singin'. just a two-fabric sandwich first.
if it sews properly, add batting to the sandwich.
if it sews properly, switch to the freemotion foot and drop the dogs. don't fiddle yet with the tension.
if it starts building a bird'snest at this point, the problem must be with the dogs. maybe they're not locked all the way down. maybe there's something gunking up their works that you can't see or get to. or ... and i really hope this isn't it ... maybe it's time to take them in for some professional "training".