I find at times like these, when you've failed after many tries, it helps to talk with another mind. I talked things out over coffee with a friend and we came up with a solution. If you follow Nophead at all you will be familiar with his plumstruder, and you will see where the inspiration came from.
I'm now surrounding the PTFE with an aluminium heat sink that does not make contact with the hot end. This keeps the PTFE cool enough to not expand, and to keep the transition zone short. I'm also using a block of wood to further insulate the hot end from the heat sink.
I've been able to run this setup for over 30 hours without fail.