OK, the problem appears to be a unhandled HTTP response error code that means that it's throwing out back to the front page without saying that an error occured (silva - if you install Fiddler you should be able to see the problem too)This only seems to occur with large(ish) amounts of text.
Um - nope, still pretty random as to *why* it happens when it does. But the error code has been discovered at the very least
a) why this has started happening - it didn't used to!
b) what I can change on the server to sort it out
At least I now know what's causing it and why it seems so strange!