Colin, the instability of emitter followers and cathode followers is quite well known, but not very well documented in text books. Some analyses crop up in the field of oscillator design.
I've been trying to find something a bit more approachable, I did find this which goes a little into the issue of gyration creating negative resistance. The action in valves is similar. It was a known problem in the valve era, but serious analysis came in the transistor era.
http://audioworkshop.org/downloads/A...T_CIRCUITS.pdf
David