Working with the machine on a thin jersey fabric the unreliable tension setting became untenable. Had previously diagnosed the issue. Last week I took apart the entire tension assembly. That's a nice thing about simple machines--it's easy to disassemble them!
Dial, washers, springs, the groove wheel and screws were set aside in the order they were removed to streamline re-assembly. Replacing the lever that clicks into the grooves on the ratchet wheel (which compresses the springs as it moves along the tension axle) was baffling. (It's the wide white disk, photo right).
Left the machine overnight and today approached it anew. Figured it out! The tension is perfect now. I've been jury rigging it for ages. This is great!