If you are using a FMQ (darning) foot rather than a walking foot, you can adjust the pressure by wrapping an elastic band around the top of the post -- push down on the spring a bit first. I have found I can actually get better straight line results with a darning foot than with a walking foot on my machine -- a 1975 Singer Genie.
The other thing I have found that helps if I do use a walking foot is to alternate between sewing horizontal & vertical lines. That way the bunches are pushed out to the corners rather than popping up at the intersections. Also, I know it's a big "no-no" in the quilting world, but sometimes I will pull behind & in front of the walking foot. Apparently, on some machines that will cause slipped or loose stitches, but I don't have that issue & it helps to compensate for some of the QC issues I have using a modern generic walking foot with my "vintage" machine.