For the sake of good order, I have felt bound to answer many of the questions that this topic prompted...
It has taken some time, because of other distractions , to actually be in a position to be able to respond: checking out aeroplane, new pup, PFA permit renewal, sufficient spinning to actually have a point of view, etc.
Q. So what happened?
A. Really I dunno
I have tried to replicate everything save from doing it over the same aerodrome but it has never happened again.
I checked the fuel filters/ injection screens initially, I have done loads of ground runs - I replaced the oil as part of my permit renewal and had yet another look at idle mixture/speed.
Tuesday this week I flicked the aeroplane into both left and right multi turn spins and had no adverse effect. Wednesday I did my permit renewal flight and did simple spins to validate - last evening I ran through the same sequence at sequence height 3 times - initially overhead just in case then elsewhere as confidence grew.
I've spoken to ONE other Pitts pilot (BA747 Captain) who years ago experienced a similar occurrence.
So the short answer is I don't know - that is a bit worrying!
Anyway this w/e will be practicing for next w/e's comp at Breighton
Stik & Reno (my pup)