Ive Just discovered this, but It's because of search cookies. The cookies in your browser have been stored once you start searching for a flight. Once they've (actually the sites you've clicked through like Orbitz, but I think not Kayak) detected that you've searched this flight a few times over the past few days, it raises the prices. Something like "this fare is no longer available". They deny they do it - do a google search on it,

But to get the original Kayak price DELETE YOUR BROWSER COOKIES before you get down to searching or buying that flight again.