Quite often you will get a leftward drift at the end of a seam, which can be cured with a good stiletto and sharp attention. I have to go back and re-sew the ends of my seams all too often! Dirt, fuzz, and oil globs will also cause this kind of problem, so your cleaning and oiling may fix the problem. I used to have a Kenmore that drifted left constantly. That may have been the worm gear, but I'm still not far along enough to know how to check that! Good luck, bkay!
PS - run a strong thread or use a stiff brush under your bobbin spring. I had one with just a large speck of dust in it that created havoc!