"GET undefined" slowing my site down

I use GTmetrix to check my site's page load times. Recently I've noticed that in the timeline, there is a GET undefined (http://*****.com/undefined) that returns a 404 error and takes up 30 seconds or more of time. Everything else loads in 1-2 seconds.

Here are the details from GTmetrix:

[TABLE="class: netInfoHeadersText netInfoText netInfoHeadersTable, width: 661"]
[TR]
[TD="class: netInfoParamName, align: right"]Content-Encoding[/TD]
[TD="class: netInfoParamValue"]gzip[/TD]
[/TR]
[TR]
[TD="class: netInfoParamName, align: right"]Vary[/TD]
[TD="class: netInfoParamValue"]Accept-Encoding, User-Agent[/TD]
[/TR]
[TR]
[TD="class: netInfoParamName, align: right"]Transfer-Encoding[/TD]
[TD="class: netInfoParamValue"]chunked[/TD]
[/TR]
[TR]
[TD="class: netInfoParamName, align: right"]Date[/TD]
[TD="class: netInfoParamValue"]Fri, 22 Mar 2013 19:02:10 GMT[/TD]
[/TR]
[TR]
[TD="class: netInfoParamName, align: right"]Server[/TD]
[TD="class: netInfoParamValue"]LiteSpeed[/TD]
[/TR]
[TR]
[TD="class: netInfoParamName, align: right"]Connection[/TD]
[TD="class: netInfoParamValue"]close[/TD]
[/TR]
[TR]
[TD="class: netInfoParamName, align: right"]X-Powered-By[/TD]
[TD="class: netInfoParamValue"]PHP/5.2.17[/TD]
[/TR]
[TR]
[TD="class: netInfoParamName, align: right"]X-Pingback[/TD]
[TD="class: netInfoParamValue"]http://*****.com/xmlrpc.php[/TD]
[/TR]
[TR]
[TD="class: netInfoParamName, align: right"]Content-Type[/TD]
[TD="class: netInfoParamValue"]text/html; charset=UTF-8[/TD]
[/TR]
[TR]
[TD="class: netInfoParamName, align: right"]Expires[/TD]
[TD="class: netInfoParamValue"]Wed, 11 Jan 1984 05:00:00 GMT[/TD]
[/TR]
[TR]
[TD="class: netInfoParamName, align: right"]Cache-Control[/TD]
[TD="class: netInfoParamValue"]no-cache, must-revalidate, max-age=0[/TD]
[/TR]
[TR]
[TD="class: netInfoParamName, align: right"]Pragma[/TD]
[TD="class: netInfoParamValue"]no-cache[/TD]
[/TR]
[TR="class: netInfoRequestHeadersTitle"]
[TD="colspan: 2"]Request Headers[/TD]
[/TR]
[TR]
[TD="class: netInfoParamName, align: right"]Host[/TD]
[TD="class: netInfoParamValue"]*****.com[/TD]
[/TR]
[TR]
[TD="class: netInfoParamName, align: right"]User-Agent[/TD]
[TD="class: netInfoParamValue"]Mozilla/5.0 (X11; Linux i686 on x86_64; rv:14.0) Gecko/20100101 Firefox/14.0.1[/TD]
[/TR]
[TR]
[TD="class: netInfoParamName, align: right"]Accept[/TD]
[TD="class: netInfoParamValue"]image/png,image/*;q=0.8,*/*;q=0.5[/TD]
[/TR]
[TR]
[TD="class: netInfoParamName, align: right"]Accept-Language[/TD]
[TD="class: netInfoParamValue"]en-us,en;q=0.5[/TD]
[/TR]
[TR]
[TD="class: netInfoParamName, align: right"]Accept-Encoding[/TD]
[TD="class: netInfoParamValue"]gzip, deflate[/TD]
[/TR]
[TR]
[TD="class: netInfoParamName, align: right"]Connection[/TD]
[TD="class: netInfoParamValue"]keep-alive[/TD]
[/TR]
[TR]
[TD="class: netInfoParamName, align: right"]Referer[/TD]
[TD="class: netInfoParamValue"]http://*****.com/[/TD]
[/TR]
[/TABLE]

How can I go about figuring out where the problem is? This is all latin to me, and I can't figure out if I'm supposed to be able to identify the issue from what GTmetrix has told me. Any advice would be appreciated.

Can't tell you much without seeing source code tbh. You got an "undefined" PHP variable somewhere down your code which then is used in some initial request.

Your header is perfectly normal. There must be something else... Did you check your error log?

Check what files are loading with firebug in Chrome or Firefox, under Network tab...it may give you an idea. You can also install DEBUG Bar plugin and check if you have any critical error caused by any plugin (which is indicated by a red color).