AD accounts not auth web users

I am in a AD forest and I have a few users that will randomly lose credentials to the Intranet web server. They are still able to log into AD, get thier mail, but when they try to access the Intranet site they are prompt with user and pass. Thier user and pass will not work, but if I remote in and use admin creds, it goes right in. The only fix I have gotten for this is blowing away thier profile and starting new… a pain for remote users, which 80% of my users are.

Anyone have an idea of the cause?