Scripts fail when jQuery.js isn't cached. When cached, scripts run fine

I have jQuery UI Tabs which load their content via AJAX. About once every 15 times when the entire page is loaded (not just XHR), things fail and I don't see the proper content in the tab.

Fiddler showed me that when things fail I also see that jQuery.js and jQuery-ui.js are both sent to the browser in full (~100kB). Normally, a page load results in HTTP status code 304 for both of those files, they're not re-downloaded, and the page displays properly. When the status code is 200 and fresh copies of jQuery/UI are sent, things fail.

I notice this most often in IE8, but that's because I use it for web development. I have seen it in Firefox, but for some reason I can't reproduce it now.

Fiddler shows that the HTTP request asks for:

GET /Scripts/jquery-1.3.2.min.js?_=1255309685187 HTTP开发者_C百科/1.1

I can't figure out what the ?_=1255309685187 is for, but I'm guessing it's a token to indicate for how long the file should be cached.

Since I can't reproduce the problem in Firefox right now, I don't know what Firebug says.

Any insight would be appreciated.

EDIT: This is with Visual Studio's development webserver.

I think you are wrong about the underscore parameter. The only way that should get there is if it is part of your script src attribute. Have you tried a javascript debugger?