dynamo light cutout

I have new shimano nexus dynahub with the switch unit that comes with
it and a B&M lumotec + with standlight
(http://www.sjscycles.com/store/item544.htm)and d toplight + rear
(http://www.sjscycles.com/store/item554.htm). The lights are wired in
parallel using heavier-than-usual-for-dynamos speaker wire with proper
soldered ends and spades etc. The light cuts out after anything from
a few seconds to a few minutes. The standlights remain on (if they
had time to charge up). If I stop the front wheel for a few seconds
and then set off again the lights work again. I cannot determine if
the length of time they work for correlates with anything (such as
length of time that the dynamo stops for) or is random. The dynamo
needs to stop for several seconds (eg braking and lifting the front
wheel whilst in motion doesn't do it) It seems as though slowing to
below the speed required to fire the light up (ie 2mph) does the same
thing but I cannot be certain of this. Switching the switch off and
on (without stopping the front wheel doesn't seem to do anything. I
*think* the key is stopping the elctrical output from the dynamo (as
opposed to some sort of mehanical effect from stoppingthe wheel) which
leads me to suspect something to do with the zener diode protection or
somesuch. IRRC both the switch unit and headlamp have zener diodes.
I have never knowingly shorted any terminals. I have had the unit up
to 40mph (although not with lights on) but dont often go much above 25
mph (peak speeds). Can anyone suggest a likely hypothesis for these
syptoms or has anyone had similar eperiences. ISTR Guy has bad words
for B&M lamps but that was mostly to do with terminal connections and
I don't *think* that is the issue here

best wishes
james