I agree with Miriam. All the machines that come to the house get a bath (sponge bath, but all jewellery (including tensioners and bobbin cases) is removed and soaked) and then their tensioners reassembled and adjusted per that article.
If you re-thread the machine, upper with a different color than the lower, I bet the upper thread is the one causing the grief... which is probably also why Miriam linked to the upper tension manual instead of the lower
Note: For anyone reading this down the road: if you do soak a bobbin case, throw it in assembled, but disassemble it after soaking and let it dry then reassemble. This way, you don't loose the little screws while it's soaking, but you don't leave it to rust together afterward.