The straight stitch can look uneven if the thread and needle size are not compatible. Dirty feed dogs can contribute to poor feeding of fabric. I remove the dogs, and pick between the teeth with an old needle or dental pik tool. Soak in alcohol, toothbrush, clean and clean again. It's amazing how much lint you can pull out of those tiny Vs.
The cam stack is not a difficult clean but is a necessary one. Take photos, observe the order. Though I don't know that a sticky cam stack will impact the tension.
Have you also done a thorough cleaning of the bobbin assembly? Removing the case, crud from under the inside lip of the race, cleaning out the slots in the top lip? This will make a world of difference in how the upper thread travels around it. If you haven't, I would try this step before playing too much with the tension.
Let us know what you find.