Here's the thing. The previous owner could have been using a completely different kind of thread and adjusted the top and bobbin tensions to suit that thread. Typically what you want to do in a situation like yours is start by loosening the top tension. You may also have to loosen the bobbin tension. This video by Jamie Wallen provides a very good demonstration about how to adjust tensions.
https://www.youtube.com/watch?v=Q1mRhcquZTM
I have used Signature and it is a good thread. However, I have found cotton threads such as Signature and King Tut shred more easily on my machine than polyester threads such as Glide and Bottom Line. You may want to try a polyester thread to see if it works more easily for you.