Machine sewing at random?

My Toyota Quil master 50 has developed problem where the the needle catches the bobbin thread when sewing and either sews to tight or to loose. I've checked tension, changed the needle, changed the bobbin. I have misplaced the instruction manual so unable to find a solution. Obviously this problem means the machine cannot sew at all. Please help, I need this machine this week. I've a lot to do!