[ILUG] odd squid problems

[Gav]

Every now and then when using IE thru squid and attaching something to a
message in hotmail or yahoo it throws up a "page cannot be
displayed" error

This has happened in both IE 5.0 and 5.5 but unfortunately  i cant 
replicate the situation so i cant keep an eye on squid log files and such
:(

It works fine most of the time. 
When it goes crazy, refreshing the page and the like doesnt work. but
turning off the use of the proxy and trying again works fine

Arg
Sorry this is so vague

Any ideas?

Thanks,
Gav