I am not sure from your description that it is the tension but if you think it is you will want to be sure it is clean and put together correctly. This link
http://www.singer-featherweight.com/...Service15.html will start you out in how to remove the tension parts for cleaning and part 16 shows them in detail with info on how to put it back together. If you are not comfortable taking the tension apart you could try running a piece of string through the disks for cleaning between them.