I did not get any replies, but I have now solved the problem of hot end temperature oscillation. I fitted a fan-shield:
http://www.thingiverse.com/thing:127932I had a simple unducted fan on the hot end before this.
I found however, using the new fan-shield and blowing cold air onto the jhead actually made it worse (the fan is wired in parallel with the heater) but reversing the fan direction and having it suck air from around the Jhead now makes the temperature very stable.
The fan shield fits a MK V jhead exactly.