To add just a bit to what Miriam posted, if and when you remove the tension unit from the machine, remove the set screw that holds it in. Push a piece of thin wire through the hole to make sure there is no debris in the hole.
Our machine had constant tension problems like yours is showing. It would not hold adjustments and when the presser foot was lowered it would get real tight.
When I took it apart I noticed some shiny metal in the hole the set screw should protrude from. It was aluminum shavings from when the set screw hole was tapped. As the screw was tightened to hold the tension unit this debris would compress and the tension would be pushed out by the pin and lever.
Once I cleaned all this debris out and reset the tension it has worked perfectly.
Joe