I have found that the Jetpack plugin causes some problems.
Tech support from them said that pixel.wp relates to Jetpack’s modules.
They let me know the following which you can check your site with to see what is loading or casuing problems:
open the inspector in Firefox and head to the “Network” Section
https://developer.mozilla.org/en-US/docs/Tools/Page_Inspector/How_to/Open_the_Inspector
I still have the same problem at different times and I have turned off most of the Jetpack modules especially the “Stats”.
Mike
