Thanks for you response BillyRay.
- Are the security settings identical on those two PCs to all other PCs that work fine?
Yes.
- Do those two PCs have any 3rd-party add-ons installed (e.g. Google toolbar, Yahoo toolbar, etc), or any internet security suites?
One of them does, one doesn't. When you reset IE to default settings, add-ons are all turned off though.
- Is it only certain users that have this issue? What happens if those users log in on a machine that works fine for someone else?
This I haven't tried. Next time the issue starts I will try to try this and let you know.
- Do those PCs have enough HDD space free to cache the images?
Yes
- What do you mean when you say you have 'reset IE'?
Internet Option -> Advanced tab -> Reset Internet Explorer settings
- Does the problem happen if you run IE in safe mode?
Again, I haven't tried this. I will let you know the next time the issue starts.