After doing all the cleaning, etc. that has already been suggested, is it possible to take it in to the local sewing repair shop for a good once-over? They will set your top and bottom tensions correctly. When you pick up the machine, however, take a good sized scrap of YOUR typical sewing fabric and thread. Sew a few seams with YOUR fabric and thread and if any further adjustments are needed, have them do it then before you take the machine back home. IF you're not comfortable doing all this yourself, that is. Good luck.