Thread slipping out of tension discs, off spring, upper lever or bobbin thread slips out of tension
That sounds frustrating! The easy things to do first are to be sure you are using good quality thread; rethread your machine, both top thread and bobbin thread; replace your needle with a new one and make sure it's inserted all the way and in the right direction (the flat side toward the back is how I insert mine); make sure you have the correct bobbin size, that the bobbin is inserted correctly, and that the bobbin thread was filled correctly (not loose or crooked).  From there, if these haven't solved your problem, you need to check your bobbin and thread tensions.  Most machines have a little screw on the bobbin case that you can loosen or tighten.  Adjusting the top thread tension is different on different machines, but I've used machines with those tension disks you mention and suggest you make sure you start out with the thread between the disks and not to the side. Then adjust the tension by tightening or loosening the disks, there is usually a thumb screw for that.  If you have the User's Guide for your machine, there should be a section about thread and bobbin tension, how to adjust them, and a table showing ranges of tensions for commonly used fabric types.  If you don't have that Guide, you may have to experiment.  You may also have to experiment every time you sew on different thickness and fabric types.  Good luck!