Wish i had something more profound to offer here, but i am stumped on this one too.
I know this seems maybe too simple, but it has helped me more times than i care to admit... have you tried totally rethreading your machine (including taking out the bobbin and putting it back in again)? Maybe your feed dogs are still up? Sometimes the tiniest piece of lint can cause the most havoc - if i am rethreading anyway it doesnt hurt to clean the most obvious parts i can see!
Sometimes you do all that and the problem magically disappears - my only complaint about that is i wish i knew exactly which one of those things i tried was what really helped
If none of the above works maybe switch out needles a couple times to see if that helps. Maybe the jeans needle and the thread combo are a mis-match? For fmq on my domestic machine i have had good results with the Microtex needles.
Hoping the problem doesnt persist too long!