squid 2.7
ubuntu server 9.04
squidguard 1.4

Squid is running sweet for 99% of the time. I have users being logged by their active directory usernames via authentication passed over by the users active directory username without any user input.
Every now and then for no apparent reason, I get a username/password prompt when browsing a site, this can be any site. I have to keep clicking on 'cancel' about 4/5 times and the user prompt just dissappears and then I carry on browsing the site. I cannot nail this issue down. Has anyone had this issue before? Next time it happens, I will make a note of the exact time and check it out on the squid access.log so I can reveal more info