Limbania, are you sure it's supposed to be DOWN??? Many sewing machines need the pressure foot UP when threading! When the foot is down, that engages the tension assembly... and can make the thread not really have room to go in between the tension disks like they are supposed to, because they are being squeezed together by the pressure applied when the foot is down. So it sort of feels like the thread is in there correctly, but it may not be, causing tension issues when sewing. Just wondering...
Cleta, what size needle are you using? Perhaps try a smaller needle for your less dense sewing (like a 9 or 11) and a thicker stronger needle for your art quilting (14 or 16). Also, have you tried changing the presser foot Pressure? On Bernina's that is often a dial on the left side of the head. It regulates how much downward pressure the foot actually applies as the fabric is being fed through the feed dogs. If it's not set correctly for the work at hand, it can mess up the stitches.
I don't know exactly what you mean by this: "I am very careful to cut the thread mid-point each time and pull the bottom half through the needle and the top half out of the guides (I think Terry White mentioned this in one of her books)." Could you clarify please?
You also say: "I am using Bottom Line almost exclusively in the bobbin no matter what I do - and lately it is snarling like a doberman." Do you mean the BL thread is now knotting or bunching or getting stuck and built up IN the bobbin assembly? Or it looks bad on the back of the quilt?
I am guessing that when you sew on the thicker, painted, embellished art quilt parts, that is putting some type of stress on the mechanisms... the bobbin assembly and the tension assembly. Perhaps it's even bending the needle or shifting the angle of the needle bar. Perhaps it needs to go back to the serviceman with examples of the problem. Also sew on those thick and thin fabrics right there in front of him.
Good luck... let us know what it was!