My best machine just began pulling the top thread through to the internal bobbin mechanism and 'hanging up'. I oil her and keep her lint free. I use good thread that she has always loved (Gutterman). I feel like it may be a tension problem, but tried to adjust. The top stitch is not her familiar quality, but she works for a while and then the top thread gets stuck in the internal bobbin mechanism. Please help. I am in the middle of piecing for a class.