American Patchwork and Quilting Magazine had this tip Dec/2008.
Sloppy machine stitches making you crazy?
There could be a simple fix. Before trying anything else, rethread your sewing machine with the presser foot RAISED.
You might be in the habit of lowering the presser foot when threading the needle (the better to see the tiny needle eye!), but if the foot is DOWN when you thread the machine, the thread probably won't be flossed properly through the machine's tension discs.
Won't be the answer to all our problems, but who knew such a small thing could make a difference?