I know this is an update to an old thread but I figure I would post the solution for anyone else researching this code. After taking it to a dealer for the second time, they told me it would be best to change both the purge valve and purge flow sensor as a set. Even though I had already changed both from junkyard cars, I figured I'd bite the bullet, and I forked over the money for some new sensors and installed them myself. Problem has been fixed since Mid-February.