Background
I work on a website that gets a fair amount of traffic, and as such, we have implemented different tracking pixels and techniques across the site for various specific reasons.

Because there are many agencies who are sending traffic our way through email campaigns, print ads and SEM, we have agreements with a variety of different outside agencies for tracking these page hits. Consequently, we have tracking pixels which span the entire site, as well as some that are on specific pages only.

We have worked to reduce the total number of pixels available on any one page, but occasionally the site is rendered close to unusable when one of these third-party tracking pixels fails to load. This is a huge difficulty on parts of the site where Javascript is needed for functionality built into the page, but is unable to initialize until a 404 is returned on the external tracking pixel. (Sometimes up to 30 seconds later)

I have spent some time attempting to research how other firms deal with this sort of instability with third-party components, but have come up a bit short. The plan currently is to implement our own stop-gap method to deal with these external outages, but rather than reinventing the wheel, we wanted to find out how this is dealt with on other sites.

Question
Is there a good set of guidelines that should be followed when implementing third-party tracking pixels?

I would love to see some white papers or other written documents about how other people have dealt with this issue.