I'm sure the tandem is asking a lot (it asks a lot of me when I pedal it), but it did pretty much the same thing on the original single mountain bike too. Even asking for fiarly modest power, say just enough to feel some assist on a hill, will make it cut out pretty quickly. This would be probably less than "half throttle", but continuously. I should think the system would be able to deliver that? It does have a lot of power when you give it "full throttle" for a brief time.