Five Tips to Solve Thread Tension Problems
Use a brush to clean under the needle plate and in the bobbin case to remove any accumulated lint. Oil if necessary.
Replace your needle. Even a new needle can be damaged. (Recommended is a Topstitch needle for decorative stitching because of its sharp point and large eye)
Re-thread your machine
Re-set the bobbin, making sure the bobbin is turning in the direction the manufacturer recommends.
Change your thread in the top and bobbin, so they are the same weight and composition, some like to use 40wt cotton for this step. Make sure you have your thread spooled correctly. Cross wound spools need to feed into your machine on your horizontal spool holder and parallel wound spools need to be mounted on your vertical spool holder with the thread unspooling from the back.
If these steps don't solve your tension problems, it's probably time for a tune-up at your local dealer.
I found this most helpful as I have several older sewing machines and some do not have a manual. I hope you do too.