iTunes kept throwing up an 8012 error (some items could not be downloaded) on some items in my queue. After a wild goose chase around the web, and messing around with the list.plist file in C:\Users\USERNAME\Music\iTunes\iTunes Music\Downloads\ I discovered it was actually related to Privoxy. Temporarily disabling it will solve the problem. Adding '.apple.com' to Privoxy's fragile sites list (under site-specific rules in the default.action file) will fix it permanently.
If you're not running Privoxy, it may be related to other proxies, firewalls or such that you may be running.