I have experienced the same thing with older machines. More than once I have cleared up this problem by installing a new "behive" spring. Stands to reason that the spring just gets tired after a whole bunch of years. Be careful when taking the tension mechanism apart, you want to be able to put the pieces back in the right order... This is a cheap fix, the spring only costs a few dollars, and it doesn't have to go to the shop, if you are reasonably handy, you can swap it out yourself like I have.