Intermittent Script error - DOTNET

I have a rather infuriating problem with what looks like an AJAX
error. We have a single server providing pages to more than one
domain. One of the domains delivers static content whilst the other
is dynamic.

The dynamic site is ASP.NET 3.5 with a fair amount of AJAX. The
server is behind a firewall and load balancer. A few (and it is a
few) of our users are getting the following script error:

Sys.ScriptLoadFailedException The script /ScriptResource.axd?
hhhhh..... failed to load.
Check for Inaccessible path

Missing call to Sys.Application.notifyScriptLoaded().

Now the majority of the users are getting the pages served correctly
but 2 are having problems and cannot use the site.

The error occurs on two different pages, one is heavy in its use of
AJAX, the other is not too heavy (few update panels - not nested and
some standard ASP.NET controls).

Has anyone seen this before? I have Googled but nothing relevant is
appearing in the search results.

Thanks in advance,
Mark