You caught the angled foot I was trying to describe.
Sewslow mentioned the site where she got her foot so I looked up ones supposedly for Singer ...link below...and that foot looks almost exactly like the one you have (and only 10 dollars). I guess you've tried about everything that you can, so if possible, buy a different one with a spring and see if it makes a difference. AGain, I like Big Foot, but there are many out there.
http://www.sewingmachinesplus.com/si...t-161876-p.php