I just noticed bitcoin-qt is using 100% CPU. It's still responsive, but it's a start...
That was probably a red herring. I was running 'bitcoin-qt -noconnect' to stop it connecting to any peers. That seems to be what causes the 100% CPU usage, whether I use your wallet or mine.
Yeah - that was unrelated. Giving bitcoin-qt the -noconnect flag causes the CPU to go to 100% for everyone, I guess. I reported it, and provided a fix:https://github.com/bitcoin/bitcoin/issues/1664
So I'm back to not being able to reproduce your problem. I doubt that broadcasting the transactions to the network would cause a problem, but it's possible. I guess I'd like to try with one of your bigger wallets once you're able to check them out.