Sporadic ASP .NET AJAX errors

I've got a website that is having very sporadic ASP.NET AJAX errors. I have a JavaScript logging solution that reports back to me when any JavaScript errors occur on my web pages. There are two errors that occur most frequently are ['Type not defined] and ['Sys' not defined]. These both are objects defined by the .NET AJAX extensions. It appears that in some cases (probably 1% or less) these types fail to initialize. The issue occurs on Internet Explorer 6, 7, and 8. The only way I have been able to reproduce this error is forcing the first ASP .NET JavaScript file to not load. Should I just chalk this up to sporadic internet hiccups? Most of the issues related to this that I've seen in my internet searches are errors that occur constantly because of misconfiguration, but my site works almost all of the time and only has issues occasionally.

The other odd error is that I have is AJAX calls back to our server that fail to return all of the data. The HTTP header indicates one size for the data, and the actual text returned is considerably smaller, anywhere from only 10% to half, and even a few that returned 0 bytes. And this is after accounting for the fact that the data is UTF8 encoded. I have always assumed that if the AJAX call returns, and readyState property is 4 and the status is 200 then the data being returned should be correct, accurate, and complete. This error seems to be across all Internet Explorer versions and I've even seen it with Firefox. Has anyone seen any issues similar to either of these, or have any suggestions as to how to debug this?