Just to make sure; you really need to go over a machine on a regular basis, all plates off; faceplate, throat plate, the round plate in the back, bobbin case, even hook and race. If you use lower priced thread (like I some times do) surprising amounts of lint will accumulate in a short time. It's often very basic, but it needs to be done regularly and thoroughly. Oil once or every other week, more if you spend hours every day on the machine. It can sound a bit tedious to get an advice like this, but it pays off in the long run. If it's been left on its' own for a while it doesn't take long to make it run smootly again.