Just wanted to add my name to the list of users who are being affected by this issue. Have repeated 800 errors, I can guarantee that if i buy a TV series, at least two episodes (maybe more) will be corrupt.
Have contacted "Support" (LOL) on this issue several times, usual story, they tell you to do what you have already tried, then refund you for the affected episodes. This of course is totally useless, as how can you watch a series with episodes missing?
I have tried all the solutions proposed in this thread, no improvement. Guess I too must also look at other ways of getting my digital content. Strangely like a few others I have had this issue since the "upgrade" to Itunes 8, never before. What is most frustrating though is the way Apple take the default route to blaming the user, and dont acknowledge there may be an issue with their server service.