If your machine is three years old, it may be that the cause of your tension problems are now because a part needs replaced. The springs in industrial machines don't last forever, and the machines are made so these parts can be replaced as needed. Thus the machines run well forever with routine maintenance. I'd replace the springs one at a time and see if your problem gets solved. Springs like: check spring, cone spring, antibacklash spring in the bobbin case. If the felt pads in the tensioner are worn out, that will cause tension problems. Also, if you've dropped your bobbin case on a hard floor, it may be slightly out of round and need replaced. Sometimes the difference can't even be detected by eye. If you get lint under the antibacklash spring in the bobbin case or under the tension arm on the bobbin case, you'll have tension issues. (All I've told you here is the voice of experience.)