Clean your upper thread path-use un-waxed dental floss to draw thru the tension disks--with presser foot raised, Clean around and under bobbin race with Q tip and brush. Almost always when bad stitches on bottom, the ''fix'' is on top, and vice versa. Re-thread top and bobbin with compatible thread. Raise the foot when adjusting tension and test after each adjustment .

Yes for the above advice. Your machine should be rethreaded and replace your needle with the correct size for the thread you are using. Needles don't last like they used to.

Check the bobbin tension.