Ajax callback not called until new tab opens in IE7 - ajax

Question

We're having some really strange problems with IE7, and only IE7. When trying to replicate the issue it only happens with native IE7, not when running IE7 mode in IE8/9, so please remember that if you try to replicate this issue.
The problem is the following:
We're polling for a response from the server with AJAX. The user posts something that the server may have to work on for a bit, so every 5 seconds or so a request is sent check if the server is done. This works fine in every browser, except native IE7. The problem is that it never stops "loading". When checking the requests with Fiddler2 we see that it does two requests and then nothing more. It stops doing anything when still in PENDING mode. In a sain browser it keeps on polling, and then stops when it gets "CONFIRMED". The really weird thing is that it does its final request and returns as normal, only, and really only, when you open a new tab.
It's not that the page needs focus or anything, clicking around randomly does nothing. I'm asking here because I can't even reproduce the issue using a local instance of the project.
Here are the headers sent back by the server:
These are the headers for the response on the production machine. This was the last response gotten. It loads indefinitely until you open a new tab (just an empty new tab!), then the final request is made and everything works out.
HTTP/1.1 200 OK
Accept-Ranges: bytes
Age: 0
Cache-Control: max-age=0, private, must-revalidate
Content-Type: application/json; charset=utf-8
Date: Tue, 04 Oct 2011 07:37:45 GMT
ETag: "867dafc628c43b6ca8a73d1977669250"
P3P: CP="ALL DSP COR CURa ADMa DEVa OUR IND COM NAV"
Server: nginx/1.0.6
Set-Cookie: _web_session=COOKIE; path=/; expires=Tue, 04-Oct-2011 10:37:45 GMT; HttpOnly
Vary: Accept-Encoding
Via: 1.1 varnish
X-Cache: MISS
X-Runtime: 0.062794
X-UA-Compatible: IE=Edge,chrome=1
X-Varnish: 55900984
Content-Length: 145
Connection: keep-alive
{"direct_publishing_settings_id":9970,"confirmed":"PENDING","errors":{},"username":"************","url":"","blog_id":44606,"platform":"blogg_se"}
These are the headers for the same request on my local server. This does not stall up the requests.
HTTP/1.1 200 OK
X-Ua-Compatible: IE=Edge
Etag: "253c934246a69c9ca821464f80f400b3"
P3p: CP="ALL DSP COR CURa ADMa DEVa OUR IND COM NAV"
Connection: Keep-Alive
Content-Type: application/json; charset=utf-8
Date: Tue, 04 Oct 2011 07:34:22 GMT
Server: WEBrick/1.3.1 (Ruby/1.8.7/2010-01-10)
X-Runtime: 0.459232
Content-Length: 137
Cache-Control: max-age=0, private, must-revalidate
Set-Cookie: _web_session=COOKIE; path=/; expires=Tue, 04-Oct-2011 10:34:22 GMT; HttpOnly
{"direct_publishing_settings_id":10,"confirmed":"PENDING","url":"","blog_id":29,"errors":{},"username":"fsasaffas","platform":"blogg_se"}
If you want to try it you can go to videofy.me, get a new account (it's really easy), when logged in go to videofy.me/blogger/settings/direct_publishing. Choose a blog platform in the first dropdown, press "activate" and write something into the username/password fields that appear, then press the green button and wait forever. After 45 seconds or a random amount of time open a new tab, and see that the request is magically finished.
I'm guessing it has to do something with IE7 cacheing the request, and then something is released if a new tab is opened. But it's just a guess, and googling returns nothing related. I'm posting here because it's so obscure that I hope someone here knows anything about it.