Martin, I was looking forward to reading the postings on this one, but 30 hours on and there aren't any! Here is my feeble effort, which has probably already occurred to you anyway ...
If the servo for the brakes is powered by inlet manifold depression, as is normal on a petrol engine, any loss of depression will not only cause the engine to lose power but also affect the brakes. I wonder if air is getting into the inlet manifold from somewhere other than the correct source? Difficult to see how this could be intermittent but it's a start.
I've just remembered something like this happening many years ago. There was a problem somewhere between the inlet manifold and the servo; I think it was either a split hose or a sticking non-return valve that caused an intermittent air intake which in turn partly or fully destroyed the vacuum.
Sorry about monopolising this thread but what I just said doesn't make sense! I don't think a sticking NRV would cause a loss of vacuum in the inlet manifold (though it would affect the functioning of the brake servo) but a split/loose hose could still be a possibility.
Like you I was waiting for someone who really knew before I thought of winging it.
Problem is with very new cars we tend to be looking for the complex tuning/ecu stuff and sometimes it is good to remember the old methods.
If the engine does cut out completely with instant loss of braking servo then a massive air leak is a possibility...but how does it seal again?
Absolutely no help to Martin but I did have similar once with a Ford Granada 2.8 when the plastic servo NRV blew out its cap and stalled the engine. We were going down a steepish hill in town with a big car trailer hitched up. It really did feel as if the brakes had failed.
Order was restored with a £1 coin which was exactly the same size as the plug and a bit of insulation tape for good measure.
David, that's what I like to see, a bit of innovation! Though I would have thought the lost cap would be the size of a penny rather than £1, given Ford's obsession with engineering economy!
I did hesitate to respond to Martin's query in case the fault was in a complex modern system because I'm not very familiar with these but I couldn't relate brake servo loss to a fault in an ECU or similar.
It wasn't absolutely clear from Martin's posting if the engine and servo actually failed completely or just went down a bit. I interpreted 'seems to lose power' as meaning only a small loss, which I thought might follow from by movement of the engine relative to the servo causing a loose or split hose to intermittently open and and re-seal. I'm sure I can remember an earlier instance of this happening, probably on one of my Triumphs!
It depends if the engine stops, and so loses manifold vacuum and thus servo effect, or if the vacuum loss causes the engine to stop.
I reckon you could be on track regarding the air leak. Yes, losing the engine will also lose the vacuum for the brake servo, but usually there's enough vacuum left in the servo for a couple of brake aplications after an engine has stopped. If the braking servo effect is lost immediately, then it's probably an air leak.
So, an analysis of the failure mode should indicate if the engine losing power or stopping is cause or effect.