I am also just using plain English (en)
I didn't have any problems with those messages, either. I am using Apache 2.2.3, PHP 5.2.0, and Courier IMAP 4.1.1. I tried it in IE7, FF2.0.0.1, and Opera 9.10. You can take down those unedited messages since they don't appear to be relevant and I'd hate for a spambot to scrape anything from them.
It's weird that it just hangs when loading the message. Usually if it hangs, there is an error getting thrown somewhere. When it "hangs" is the browser still acting as it if it loading the page/message? Do you get an hourglass?
Any chance you can monitor the connection with tcpflow to see if the data is stopping mid-message or if it's all getting sent but the browser isn't showing it all?
Not that I could see how a 30k message would come near it, but have you tried raising PHP's memory limit to something higher, say 64mb?